test(json): pin s64 MIN/MAX writer bytes; move scratch to .sx-tmp

Close the coverage gap from attempt 1: example 0713 now builds integer
fields holding s64 MIN (-9223372036854775808) and s64 MAX
(9223372036854775807) — plus zero, a small negative, and a small positive —
and asserts the EXACT emitted bytes. This permanently pins the edge that
write_int is specifically engineered for (folding positives into negative
space so MIN's non-representable-positive magnitude serializes correctly).

s64 MIN is expressed as (0 - 9223372036854775807 - 1) because its magnitude
is not a representable positive s64 literal.

Test hygiene: stream to a repo-local, gitignored .sx-tmp/ path (created if
missing) instead of a fixed /tmp name, and unlink it right after read-back
so nothing leaks. Writer/model logic and src/ are untouched.

examples/0713-modules-json-writer.sx

@@ -2,8 +2,10 @@
 //
 // Builds a representative value — a nested object holding a string with
 // every escape kind (quote, newline, tab, backslash, a raw control byte),
-// a negative integer, a bool, null, an array, and a nested object — then
+// integers spanning zero / a small negative / a small positive / s64 MIN
-// serializes it two ways and asserts the EXACT bytes:
+// (-9223372036854775808) / s64 MAX (9223372036854775807), a bool, null, an
+// array, and a nested object — then serializes it two ways and asserts the
+// EXACT bytes:
 //
 //   1. into a caller-owned `[]u8` buffer (returns bytes written),
 //   2. streaming straight to a file through an 8-byte staging buffer
@@ -20,7 +22,7 @@
 #import "modules/fs.sx";
 
 // The exact document the writer must produce (insertion order, escaping).
-EXPECT :: "{\"name\":\"a\\\"b\\n\",\"tab\":\"x\\ty\",\"bs\":\"c\\\\d\",\"ctrl\":\"\\u0001\",\"n\":-7,\"ok\":true,\"nil\":null,\"xs\":[1,-2,3],\"nested\":{\"k\":\"v\"}}";
+EXPECT :: "{\"name\":\"a\\\"b\\n\",\"tab\":\"x\\ty\",\"bs\":\"c\\\\d\",\"ctrl\":\"\\u0001\",\"n\":-7,\"zero\":0,\"pos\":7,\"min\":-9223372036854775808,\"max\":9223372036854775807,\"ok\":true,\"nil\":null,\"xs\":[1,-2,3],\"nested\":{\"k\":\"v\"}}";
 
 report :: (label: string, ok: bool) {
     if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }
@@ -48,7 +50,13 @@ build :: (alloc: Allocator) -> Value {
     obj.put("tab",  .str("x\ty"),   alloc);    // tab
     obj.put("bs",   .str("c\\d"),   alloc);    // backslash
     obj.put("ctrl", .str(ctrl),     alloc);    // raw control byte -> 

-    obj.put("n",    .int_(0 - 7),   alloc);    // negative int
+    obj.put("n",    .int_(0 - 7),   alloc);    // small negative int
+    obj.put("zero", .int_(0),       alloc);    // zero
+    obj.put("pos",  .int_(7),       alloc);    // small positive int
+    // s64 MIN: its magnitude (9223372036854775808) is not a representable
+    // positive s64 literal, so build it from MAX-positive minus one.
+    obj.put("min",  .int_(0 - 9223372036854775807 - 1), alloc);
+    obj.put("max",  .int_(9223372036854775807),         alloc);  // s64 MAX
     obj.put("ok",   .bool_(true),   alloc);
     obj.put("nil",  .null_,         alloc);
     obj.put("xs",   .array(xs),     alloc);
@@ -78,8 +86,11 @@ main :: () -> ! {
     report("overflow-raised", oerr == error.Overflow);
 
     // 3. Stream to a file through a tiny staging buffer (forces flushes);
-    //    read it back and assert it equals the same document.
+    //    read it back and assert it equals the same document. Write into the
-    path := "/tmp/sx_0713_json.json";
+    //    repo-local, gitignored scratch dir and unlink afterwards so nothing
+    //    leaks and concurrent runs don't fight over a shared /tmp name.
+    if !create_dir_all(".sx-tmp") { print("mkdir: FAIL\n"); return; }
+    path := ".sx-tmp/sx_0713_json.json";
     fh := open_file(path, .write);
     if fh == null { print("open: FAIL\n"); return; }
     f := fh!;
@@ -88,8 +99,8 @@ main :: () -> ! {
     f.close();
 
     back := read_file(path);
+    delete_file(path);
     if back == null { print("file-read: FAIL\n"); return; }
     report("file-exact", back! == EXPECT);
-    delete_file(path);
     return;
 }

examples/expected/0713-modules-json-writer.stdout

@@ -1,4 +1,4 @@
-doc: {"name":"a\"b\n","tab":"x\ty","bs":"c\\d","ctrl":"\u0001","n":-7,"ok":true,"nil":null,"xs":[1,-2,3],"nested":{"k":"v"}}
+doc: {"name":"a\"b\n","tab":"x\ty","bs":"c\\d","ctrl":"\u0001","n":-7,"zero":0,"pos":7,"min":-9223372036854775808,"max":9223372036854775807,"ok":true,"nil":null,"xs":[1,-2,3],"nested":{"k":"v"}}
 buffer-exact: ok
 buffer-len: ok
 overflow-raised: ok