wcc/ww: reject duplicate top-level decls, aligned to cstage

installdecl routes all four kinds (fn/type/def/let) through installtop,
which turns scopedefineinmodule's nil return into cstage's exact
"duplicate <kind> <name>" reject, keyed (name,mod) so cross-package
same-leaf decls coexist. Builtin redecls are dropped, not dup-errored:
cstage never scopes builtins (lookup_builtin first, check.c:69), so a
user redecl is dead there — wwstage mirrors via scopesamekeysym +
no-source-decl test. -T synth __wwtests installs direct, mirroring
check.c:3079. Closes the silent dup-fn hole (user fn run vs lib/test
run built a broken test binary with no diagnostic). Per-kind reject
rows + cross-package/builtin accept byte-id rows + -T collision parity
row in 910/997. (#23-team, category-A addendum closed)

test/wcc/data/attest_userrun.ww

@@ -0,0 +1,9 @@
+// #23 -T collision — a user `fn run` collides with lib/test's bound
+// runner `run` (the synth's callee) once `ww test -c` bundles lib/test.
+// Both stages must loud-reject "duplicate fn run" (rc!=0), never silently
+// build a binary that calls the wrong run and skips every @test.
+fn run() void = { return; };
+
+@test fn t_one() void = {
+	assert(1 == 1);
+};