wcc: -T test-mode collects @test fns + synthesizes entry, both stages (#15)

@test was parsed then dropped (no consumer); `ww test` needed a hand-written
main listing each test by hand, so adding a @test and forgetting the call
silently skipped it. -T makes the checker collect @test N_FNDECLs in source
order, loud-reject a user main, and append a synthetic
`export fn main() i32 { t0(); ...; return 0; }` at the install->body-check seam;
the existing cgfn emits it (cgen untouched) -> byte-identical by construction.
Mirrors harec's checker-side is_test placement.

Plan-9-lean reduction (user-sanctioned, reinstatable post-CSP): sequential,
abort/nonzero=fail; no setjmp isolation, no fnmatch filter, no file:line.

910/997 rewired from a regex scanner to driving `w6c -T` directly (thin trusted
drivers; the @test content stays ww), with a cross-stage byte-id assert on the
-T output. attest_userman/attest_badsig pin the user-main and bad-signature
rejects.

Makefile

@@ -2128,7 +2128,7 @@ $(BIN)/test_dyn_ww: test/wcc/996_dyn_ww.c $(BIN)/ww $(BIN)/w6l $(BIN)/w6l_ww \
 	$(CC) $(CFLAGS) -o $@ $<
 
 $(BIN)/test_at_test_ww: test/wcc/997_at_test_ww.c $(BIN)/ww_ww $(BIN)/w6c_ww \
-		$(BIN)/w6a_ww $(BIN)/w6l_ww $(LIB)/libwwrt.a | $(BIN)
+		$(BIN)/w6a_ww $(BIN)/w6l_ww $(BIN)/w6c $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
 $(BIN)/test_fmt_run: test/wcc/970_fmt_run.c $(BIN)/ww $(BIN)/w6c \