F1.2: std.hash zero-heap [64]u8 hex API + chunked file + pinned vectors

Make the SHA-256 digest path allocation-free (foundation heap-discipline):

- final() and sha256_hex() now return the 64-char lowercase hex digest as
  a [64]u8 by value on the stack; the cstring(64) heap allocation is gone.
- sha256_file() streams the file in fixed 64KB stack chunks via open_file/
  File.read/File.close (defer-closed on every path) instead of slurping it
  with read_file; peak memory is O(chunk), not O(filesize).

Tests (compare via a zero-copy string view over the [64]u8):
- 0710 updated to the by-value API (output unchanged).
- 0711 known-answer vectors: "", "abc", NIST-56/112, padding boundaries
  {0,55,56,57,63,64,65,119,120}, and 1000 / 1,000,000 'a' repeats, each
  pinned to its published digest (cross-checked with shasum -a 256).
- 0712 streaming equivalence (one-shot == byte-at-a-time == split-mid-block
  == split-on-boundary) plus sha256_file(temp) == in-memory digest.

src/ untouched. zig build && zig build test && tests/run_examples.sh green.

examples/0710-modules-sha256.sx

@@ -3,15 +3,19 @@
 // Known-answer vectors (empty, "abc", the 112-byte NIST multi-block
 // vector) plus the streaming invariant: feeding the same bytes in
 // several `update` chunks yields the same digest as the one-shot call.
+//
+// The digest is a zero-heap `[64]u8` returned by value; tests build a
+// `string` view over it (no copy) to compare against the pinned hex.
 
 #import "modules/std.sx";
 #import "modules/std/hash.sx";
 
-check :: (label: string, got: string, want: string) {
-    if got == want {
+check :: (label: string, got: [64]u8, want: string) {
+    view := string.{ ptr = @got[0], len = 64 };
+    if view == want {
         print("{}: ok\n", label);
     } else {
-        print("{}: FAIL got {} want {}\n", label, got, want);
+        print("{}: FAIL got {} want {}\n", label, view, want);
     }
 }
 
@@ -31,7 +35,6 @@ main :: () {
     h.update("abcdefghbcdefghicdefghijdefghijke");   // 33 bytes
     h.update("fghijklfghijklmghijklmnhijklmno");      // crosses block edge
     h.update("ijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu");
-    streamed := h.final();
-    check("stream-eq-oneshot",
-        if streamed == sha256_hex(multi) then "yes" else "no", "yes");
+    check("stream-eq-oneshot", h.final(),
+        "cf5b16a778af8380036ce59e7b0492370b249b11e8f07a51afac45037afee9d1");
 }

examples/0711-modules-sha256-vectors.sx

@@ -0,0 +1,82 @@
+// SHA-256 known-answer vectors for `modules/std/hash.sx`.
+//
+// Pins published digests for: the empty input, "abc", the two NIST
+// multi-block sample vectors, the padding/length boundaries around the
+// 56-byte (one-block-with-length) and 64-byte (block) edges, and the
+// classic large repeats (1000 and 1,000,000 'a'). Each expected hex is
+// hard-coded from FIPS 180-4 / NIST CAVP and cross-checked with
+// `shasum -a 256`.
+//
+// The digest is a zero-heap `[64]u8`; we compare it via a `string` view
+// (no copy). Repeat vectors are built by streaming an 'a'-filled stack
+// buffer, so even the 1,000,000 case allocates nothing on the heap.
+
+#import "modules/std.sx";
+#import "modules/std/hash.sx";
+
+check :: (label: string, got: [64]u8, want: string) {
+    view := string.{ ptr = @got[0], len = 64 };
+    if view == want {
+        print("{}: ok\n", label);
+    } else {
+        print("{}: FAIL got {} want {}\n", label, view, want);
+    }
+}
+
+// Digest of `total` bytes of 'a', streamed in 1000-byte chunks so peak
+// memory stays O(chunk) for the 1,000,000 case. `total == 0` yields the
+// empty-input digest (the loop body never runs).
+hash_a :: (total: s64) -> [64]u8 {
+    chunk : [1000]u8 = ---;
+    k := 0;
+    while k < 1000 { chunk[k] = 97; k += 1; }   // 97 = 'a'
+
+    h := init();
+    remaining := total;
+    while remaining > 0 {
+        take := if remaining < 1000 then remaining else 1000;
+        h.update(string.{ ptr = @chunk[0], len = take });
+        remaining -= take;
+    }
+    h.final()
+}
+
+main :: () {
+    check("empty", sha256_hex(""),
+        "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
+    check("abc", sha256_hex("abc"),
+        "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+
+    // NIST CAVP sample vectors (56-byte and 112-byte multi-block).
+    check("nist-56", sha256_hex("abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"),
+        "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1");
+    check("nist-112", sha256_hex("abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu"),
+        "cf5b16a778af8380036ce59e7b0492370b249b11e8f07a51afac45037afee9d1");
+
+    // Padding/length boundaries around the 56- and 64-byte edges, using
+    // 'a' repeats so the boundary is exercised independently of content.
+    check("len-0",   hash_a(0),
+        "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
+    check("len-55",  hash_a(55),
+        "9f4390f8d30c2dd92ec9f095b65e2b9ae9b0a925a5258e241c9f1e910f734318");
+    check("len-56",  hash_a(56),
+        "b35439a4ac6f0948b6d6f9e3c6af0f5f590ce20f1bde7090ef7970686ec6738a");
+    check("len-57",  hash_a(57),
+        "f13b2d724659eb3bf47f2dd6af1accc87b81f09f59f2b75e5c0bed6589dfe8c6");
+    check("len-63",  hash_a(63),
+        "7d3e74a05d7db15bce4ad9ec0658ea98e3f06eeecf16b4c6fff2da457ddc2f34");
+    check("len-64",  hash_a(64),
+        "ffe054fe7ae0cb6dc65c3af9b61d5209f439851db43d0ba5997337df154668eb");
+    check("len-65",  hash_a(65),
+        "635361c48bb9eab14198e76ea8ab7f1a41685d6ad62aa9146d301d4f17eb0ae0");
+    check("len-119", hash_a(119),
+        "31eba51c313a5c08226adf18d4a359cfdfd8d2e816b13f4af952f7ea6584dcfb");
+    check("len-120", hash_a(120),
+        "2f3d335432c70b580af0e8e1b3674a7c020d683aa5f73aaaedfdc55af904c21c");
+
+    // Large repeats (FIPS 180-4 / classic vectors).
+    check("a-1000",    hash_a(1000),
+        "41edece42d63e8d9bf515a9ba6932e1c20cbc9f5a5d134645adb5db1b9737ea3");
+    check("a-1000000", hash_a(1000000),
+        "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0");
+}

examples/0712-modules-sha256-streaming.sx

@@ -0,0 +1,76 @@
+// SHA-256 streaming-equivalence + file hashing for `modules/std/hash.sx`.
+//
+// The chunk boundary must not affect the result: feeding the same bytes
+// one-shot, one byte at a time, split mid-block, and split exactly on a
+// 64-byte block boundary all yield the same digest, anchored to a pinned
+// value. Then `sha256_file` of a written temp file must equal the
+// in-memory digest of the same bytes — the streaming file path agrees
+// with the buffered path.
+//
+// All comparisons go through `string` views over the zero-heap `[64]u8`
+// digests; the byte/split updates view directly into the input buffer
+// (no `substr`, no copies).
+
+#import "modules/std.sx";
+#import "modules/std/hash.sx";
+#import "modules/fs.sx";
+
+// 112-byte NIST multi-block vector — long enough that a 64-byte split is
+// a genuine block boundary and a 30-byte split lands mid-block.
+MSG :: "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu";
+PIN :: "cf5b16a778af8380036ce59e7b0492370b249b11e8f07a51afac45037afee9d1";
+
+check :: (label: string, got: [64]u8, want: string) {
+    view := string.{ ptr = @got[0], len = 64 };
+    if view == want {
+        print("{}: ok\n", label);
+    } else {
+        print("{}: FAIL got {} want {}\n", label, view, want);
+    }
+}
+
+report :: (label: string, ok: bool) {
+    if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }
+}
+
+// Absorb `data` one byte at a time (views of length 1 into the buffer).
+stream_by_byte :: (data: string) -> [64]u8 {
+    h := init();
+    i := 0;
+    while i < data.len {
+        h.update(string.{ ptr = @data[i], len = 1 });
+        i += 1;
+    }
+    h.final()
+}
+
+// Absorb `data` as two updates split at `at` (views into the buffer).
+stream_split :: (data: string, at: s64) -> [64]u8 {
+    h := init();
+    h.update(string.{ ptr = @data[0],  len = at });
+    h.update(string.{ ptr = @data[at], len = data.len - at });
+    h.final()
+}
+
+main :: () {
+    check("oneshot-pinned",    sha256_hex(MSG), PIN);
+    check("byte-at-a-time",    stream_by_byte(MSG), PIN);
+    check("split-mid-block",   stream_split(MSG, 30), PIN);   // 30: mid first block
+    check("split-on-boundary", stream_split(MSG, 64), PIN);   // 64: exact block edge
+
+    // sha256_file (streaming) must equal the in-memory digest.
+    path := "/tmp/sx_0712_stream.bin";
+    if !write_file(path, MSG) { print("file-write: FAIL\n"); return; }
+
+    maybe := sha256_file(path);
+    if maybe == null { print("file-eq-memory: FAIL (open)\n"); return; }
+    file_digest := maybe!;
+    mem_digest := sha256_hex(MSG);
+
+    fv := string.{ ptr = @file_digest[0], len = 64 };
+    mv := string.{ ptr = @mem_digest[0],  len = 64 };
+    report("file-eq-memory", fv == mv);
+    check("file-pinned", file_digest, PIN);
+
+    delete_file(path);
+}

examples/expected/0711-modules-sha256-vectors.exit

@@ -0,0 +1 @@
+0

examples/expected/0711-modules-sha256-vectors.stderr

@@ -0,0 +1 @@
+

examples/expected/0711-modules-sha256-vectors.stdout

@@ -0,0 +1,15 @@
+empty: ok
+abc: ok
+nist-56: ok
+nist-112: ok
+len-0: ok
+len-55: ok
+len-56: ok
+len-57: ok
+len-63: ok
+len-64: ok
+len-65: ok
+len-119: ok
+len-120: ok
+a-1000: ok
+a-1000000: ok

examples/expected/0712-modules-sha256-streaming.exit

@@ -0,0 +1 @@
+0

examples/expected/0712-modules-sha256-streaming.stderr

@@ -0,0 +1 @@
+

examples/expected/0712-modules-sha256-streaming.stdout

@@ -0,0 +1,6 @@
+oneshot-pinned: ok
+byte-at-a-time: ok
+split-mid-block: ok
+split-on-boundary: ok
+file-eq-memory: ok
+file-pinned: ok