F1.1: std.hash — streaming SHA-256 in library/modules/std/hash.sx

Add a pure-sx streaming SHA-256 (FIPS 180-4) stdlib module, importable
as `#import "modules/std/hash.sx";`. All 32-bit word arithmetic is done
in s64 and masked back with `& MASK32`, so digests are deterministic and
platform-independent — no shelling out, no native crypto.

API:
- init() -> Sha256          (by-value *self pattern)
- update(*Sha256, string)   (multi-block + partial-block buffering)
- final(*Sha256) -> string  (32-byte digest as lowercase hex)
- sha256_hex(string) -> string             (one-shot)
- sha256_file([:0]u8) -> ?string           (digest of a file via fs.read_file)

Verified against FIPS/NIST known-answer vectors and `shasum -a 256`:
"" , "abc", the 56- and 112-byte multi-block vectors, 1000×'a', and the
64/65-byte block boundaries; chunked update() matches the one-shot call.

examples/0710-modules-sha256.sx pins the KAT vectors + the streaming
invariant; gate green (zig build, zig build test, run_examples 370/0/0/0).

examples/expected/0710-modules-sha256.exit

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

examples/expected/0710-modules-sha256.stderr

@@ -0,0 +1 @@
+

examples/expected/0710-modules-sha256.stdout

@@ -0,0 +1,4 @@
+empty: ok
+abc: ok
+multi: ok
+stream-eq-oneshot: ok