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.

test/wcc/data/attest_badsig.ww

@@ -0,0 +1,18 @@
+// #15 negative fixture: an @test fn with a non-`fn() void` signature.
+// The collector must loud-reject (nonzero exit) rather than silently
+// skip or coerce — rule-7. Pins the signature guard on both stages.
+//
+// TEETH: a non-void RETURN with NO params is the case ONLY the explicit
+// guard catches. The synth entry emits `bad();` (arg count matches 0
+// params) and a discarded non-void return is silently allowed, so
+// without the guard this compiles clean (exit 0). A param-bearing @test
+// would instead trip the checker's independent "not enough arguments"
+// (check.c:1879) on the synth `bad()` call — so a param fixture would
+// pass the reject probe whether or not the guard exists, isolating
+// nothing. Hence the no-param / non-void-return shape.
+
+package data;
+
+@test fn bad() i32 = {
+	return 0;
+};

test/wcc/data/attest_userman.ww

@@ -0,0 +1,17 @@
+// #15 negative fixture: @test fns PLUS an explicit user `main`. Under
+// `-T` the entry is synthesized, so a hand-written main collides — both
+// stages MUST loud-reject (nonzero exit), mirroring harec's hosted-main
+// suppression under is_test (ref/harec/src/check.c:4000).
+
+package data;
+
+@test fn check_ok() void = {
+	let a: i32 = 1;
+	if (a != 1) {
+		let _: i32 = 1 / 0;
+	};
+};
+
+export fn main() i32 = {
+	return 0;
+};