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/dup_fn.ww

@@ -0,0 +1,5 @@
+// #23 — two top-level fns of the same name in the same (flat) module.
+// cstage's install pass rejects "duplicate fn foo"; wwstage now mirrors.
+fn foo() i32 = { return 1; };
+fn foo() i32 = { return 2; };
+export fn main() i32 = { return foo(); };