lib/memio: fixedwrite returns nomem on full buffer (F-R)

memio.fixedwrite returned a successful 0-byte write once the sink
filled, so an overflowing fprintf/bsprintf surfaced a truncated prefix
as a successful str instead of an error. Hare's fixed_write returns
nomem there (ref/hare/memio/stream.ha:161); the bsprintf/fprintf
io.error arm already forwards it, so the prefix-on-overflow path is the
only divergence.

Mirror Hare's full guard order: an empty input buf short-circuits to 0
(stream.ha:157) before the full-sink nomem guard, so a 0-byte write to
a full sink stays 0 (no new divergence). fmt.bsprintf/formatone keep
their logic; only their now-stale WHY-comments are rewritten, and
formatone's tail-pad counter is left as-is (the width-form restore is a
deferred follow-up, out of F-R scope). memio's own `fixed` doc comment,
which still claimed ww surfaces 0 on a full buffer, is corrected to the
new nomem contract.

Tests: flip the two fmt rows that pinned the prefix bug (bsprintf_trunc,
bsprintf_width_trunc) plus memiotest fixedwritecases' overflow row to
assert `is nomem`; add positive controls (bsprintf_exact must still
succeed) + an empty-sink discriminator (bsprintf_empty) + a dedicated
fixedwritefull unit pinning the memio.ww:190 contract.

Regenerates the w6c and wwdump combined.ww (memio's fixedwrite change
and `fixed` doc comment are the only embedded changes; fmt is
dead-code-eliminated from both).

lib/fmt/fmttest.ww

@@ -452,6 +452,19 @@ fn errsource() io.stream = {
 	};
 };
 
+// Positive control: an EXACT-fit render must still succeed — the sink
+// fills to the last byte but no write lands on a full sink, so the
+// nomem guard (memio.ww:190) must not fire. Guards against the
+// truncation fix over-rejecting a render that exactly fits.
+@test fn bsprintf_exact() void = {
+	let buf: [5]u8;
+	let r: (str | io.error) = fmt.bsprintf(buf[0:5], "hello");
+	match (r) {
+	case let s: str => { assert(!(!streq(s, "hello"))); };
+	case let eioe: io.error => abort();
+	};
+};
+
 // Pins the formatfield bool / rune arms inside fprintf's for-loop —
 // the codegen-smell repro shape (task #18). Pre-task-#18, the str arm
 // is also exercised by fprintf_implicit; this adds the two remaining
@@ -470,30 +483,43 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
-// Verifies bsprintf's documented truncation contract — short writes
-// return the prefix that fit (memio.fixedwrite returns 0 once full,
-// not io.error). Pairs with fprintfixedshort above which covers the
-// underlying fprint path.
+// Verifies bsprintf's truncation contract — overflowing a fixed sink
+// surfaces nomem, not a prefix (memio.fixedwrite returns nomem once
+// full, ref/hare/fmt/wrappers.ha:50). Pairs with fprintfixedshort
+// above which covers the partial-write (short i32) path.
 @test fn bsprintf_trunc() void = {
 	let buf: [3]u8;
 	let r: (str | io.error) = fmt.bsprintf(buf[0:3], "{} {}", "hi", 42i64);
 	match (r) {
-	case let s: str => { assert(!(!streq(s, "hi "))); };
-	case let eioe: io.error => abort();
+	case let s: str => abort();
+	case let eioe: io.error => { assert(eioe is nomem); };
 	};
 };
 
-// Pins formatone's tail-pad counter shape. Without the counter, the
-// `total < m.width` loop spins on memio.fixed's 0-byte-on-full
-// partial-write contract (ww divergence from Hare's errors::overflow
-// + `?`). Buffer 3B, target "   42" (5B); first 3 spaces fit, then
-// formatraw + tail-pad both write 0 — must terminate.
+// Truncation through the width/pad path also surfaces nomem. Buffer
+// 3B, target "   42" (5B): the 3 lead pad spaces fill the sink, then
+// formatraw writes "42" into the full sink → nomem. Also pins
+// formatone's tail-pad counter (the loop must not spin on the full
+// sink before the error propagates).
 @test fn bsprintf_width_trunc() void = {
 	let buf: [3]u8;
 	let r: (str | io.error) = fmt.bsprintf(buf[0:3], "{:5}", 42i64);
 	match (r) {
-	case let s: str => { assert(!(!streq(s, "   "))); };
-	case let eioe: io.error => abort();
+	case let s: str => abort();
+	case let eioe: io.error => { assert(eioe is nomem); };
+	};
+};
+
+// Empty-sink discriminator: the cleanest single-write red/green split.
+// The first non-empty write to a zero-length sink hits the full-sink
+// guard → nomem; pre-fix it returned 0 and bsprintf yielded "" as a
+// success.
+@test fn bsprintf_empty() void = {
+	let buf: [0]u8;
+	let r: (str | io.error) = fmt.bsprintf(buf[0:0], "x");
+	match (r) {
+	case let s: str => abort();
+	case let eioe: io.error => { assert(eioe is nomem); };
 	};
 };