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.

selfhost/cmd/w6c/main.ww

@@ -79,6 +79,7 @@ fn slurp(path: *u8) (*u8, u64) = {
 export fn main(argc: i32, argv: **u8) i32 = {
 	let src: *u8 = nil;
 	let out: *u8 = nil;
+	let testmode: i32 = 0i32;	// #15: `-T` test-mode
 
 	let i: i32 = 1;
 	for (i < argc) {
@@ -91,6 +92,8 @@ export fn main(argc: i32, argv: **u8) i32 = {
 				return 2;
 			};
 			out = argv[i];
+		} else { if (cstreq(a, "-T")) {
+			testmode = 1i32;
 		} else { if (a[0u64] == 45u8) {
 			let m: str = "w6c: unknown flag\n";
 			os.write(2, m.ptr, m.len: u64);
@@ -102,12 +105,12 @@ export fn main(argc: i32, argv: **u8) i32 = {
 				return 2;
 			};
 			src = a;
-		}; };
+		}; }; };
 		i += 1;
 	};
 
 	if (src == nil) {
-		let m: str = "usage: w6c_ww [-o out.s] file.ww\n";
+		let m: str = "usage: w6c_ww [-T] [-o out.s] file.ww\n";
 		os.write(2, m.ptr, m.len: u64);
 		return 2;
 	};
@@ -168,6 +171,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	typesinit(&tc);
 	let ck: checker;
 	checkinit(&ck, &tc);
+	ck.istest = testmode;
 	checkfile(&ck, f);
 	if (ck.errs > 0) { return 1; };