lib/crypto/sha256: port SHA-256 over hash::hash; NIST test (989)

Port of ref/hare/crypto/sha256/sha256.ha — block-processed [64]u8
chunks, u32 modular arithmetic, hash::hash + io.writer surface. The
state embeds hash.hash (inline vtable at offset 0); the vtable + sum/
reset slots are wired post-construction (base64/memio convention).

u32 WRAPPING + vtable dispatch CONFIRMED CLEAN: all NIST vectors verify
byte-identical — empty, "abc", the 56-byte block-boundary case, and the
one-million-'a' multi-block stream (1000-byte chunks across many blocks,
stressing write()'s partial-block carry). cgen truncates u32 add/shift/
rotate to 32 bits correctly; no masking workaround needed.

Semantics-preserving spelling divergences (slice-copy as byte loops,
close()/digest loops) are noted at-site per CLAUDE.md rule 5/13.

ONE BEHAVIORAL DIVERGENCE, blocked on a cgen bug (flagged for ken/drew):
Hare's sum() snapshots the state (`let copy = *h`) so it is re-entrant.
That deref-copy of an array-containing struct miscompiles in ww cgen
(copied array fields come back zeroed). So sum() runs on the live state
and is SINGLE-SHOT until the cgen fix lands; every current caller does
one terminal sum(), so the digests are unaffected. Minimal repro:
  type t = struct { h: [4]u32 };
  let c: t = *(&s);   // c.h reads back wrong
A sibling bug (array return-by-value zeroes the result) was also found
and is avoided in the test's buffer-based helper. Both filed for ken.

The hash/crypto modules are dead-imported (no selfhost combined.ww
regen). 9xx test numbers are full, so the run-test shares the 989
prefix with siphash (distinct `short` name; 949_* multi-file precedent).

Makefile

@@ -396,7 +396,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_base32_run $(BIN)/test_base64_run \
 	  $(BIN)/test_adler32_run $(BIN)/test_crc16_run \
 	  $(BIN)/test_crc32_run $(BIN)/test_crc64_run \
-	  $(BIN)/test_siphash_run \
+	  $(BIN)/test_siphash_run $(BIN)/test_sha256_run \
 	  $(BIN)/test_checked_run \
 	  $(BIN)/test_floatarr_run \
 	  $(BIN)/test_deref_narrow_run \
@@ -1565,6 +1565,10 @@ $(BIN)/test_siphash_run: test/wcc/989_siphash_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_sha256_run: test/wcc/989_sha256_run.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_bufio_run: test/wcc/998_bufio_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<