test/989: lib byte-id gate — w6c vs w6c_ww over every non-embedded lib unit

The 990-997 gates byte-id only the selfhost-embedded modules; every
other lib/ module compiled cstage-only, which let regex.finish ship
cs≠ww for weeks (task #21, FC0). 989_lib_byteid compiles each lib test
fixture's resolved unit (plus import-probe stubs for the fixtureless
sort/path/endian/net/hash/fnv/crypto.math/c.libc) through both stages
and byte-compares the asm: 28 units pinned byte-identical (incl.
lib/regex), 12 known divergences + 3 wwstage front-end rejects pinned
as documented-allowed with task #59 cites — a landed fix trips the pin
and demands graduation, so the corpus can only ratchet toward ID.

Two rot-guards, both review-driven: each probe carries a sentinel that
must appear in the resolved unit (the driver silently skips an
unresolvable import, so a dropped probe would byte-id an empty main —
green while covering nothing), and a corpus-completeness scan fails
loudly on any lib/ dir not enrolled, so new modules cannot ship
uncovered.

Compile+cmp only (no driver run, no source-tree writes): phase-1
parallel-safe, ~6s.

Makefile

@@ -422,6 +422,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_crc32_run $(BIN)/test_crc64_run \
 	  $(BIN)/test_siphash_run $(BIN)/test_sha256_run \
 	  $(BIN)/test_regex_run \
+	  $(BIN)/test_lib_byteid \
 	  $(BIN)/test_checked_run \
 	  $(BIN)/test_floatarr_run \
 	  $(BIN)/test_deref_narrow_run \
@@ -1790,6 +1791,10 @@ $(BIN)/test_regex_run: test/wcc/989_regex_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_lib_byteid: test/wcc/989_lib_byteid.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6c_ww $(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 $@ $<