cstage+selfhost+test: mangle fn labels by module (#9)

Both stages emitted fn TEXT labels by leaf only; lib/os and lib/io
exporting the same leaves (read, write, close) collided at link.
lib/fmt + lib/log worked around with @symbol("rt_syscall") stubs.

Drop d->export from the fn skip rule in mod_collect (both stages) so
exported fns mangle as <module>.<name>. Let/def/type keep current
behavior. Skip retained for {@symbol, main, empty-module}.

Add cur_mod thread through cgfn + mod_lookup_for_fn(name, hint) at
all 4 label-emit sites (TEXT def, LEAQ N_IDENT, CALL N_IDENT, CALL
N_DOT). Wwstage mirror: emitfnname + modlookupforfn + curmod.
Invariant comment pinned in both stages.

ww2 == ww3 == ww4 byte-identical at the new label format.
706_fnlabel_mangle covers same-leaf cross-module CALL + private-leaf
cur_mod disambiguation through a fn-pointer rvalue.

Wwstage LEAQ-of-fn N_DOT (`let p = mod.fn` rvalue) is a pre-existing
gap; deferred to a follow-up. fmt/log rt_syscall stubs untouched
here; cleanup follows.

test/wcc/data/fnlabelmangle/mod1/mod1.ww

@@ -0,0 +1,19 @@
+// Two modules each `export fn ping` and each define a private
+// `helper` with the same leaf. Together with mod2 they pin three of
+// the four label-emit sites #9 touches:
+//   - CALL N_DOT      : main calls mod1.ping / mod2.ping
+//   - CALL N_IDENT    : ping bare-calls helper inside its own module
+//   - LEAQ N_IDENT    : fpi takes `helper` by value, then calls it
+//
+// The LEAQ N_DOT path (`let p = mod1.ping`) is intentionally not
+// exercised here — wwstage cgdot has no working LEAQ-of-fn N_DOT
+// branch, so a row would diverge across stages.
+
+fn helper() i32 = { return 11i32; };
+
+fn fpi() i32 = {
+	let h: fn() i32 = helper;
+	return h();
+};
+
+export fn ping() i32 = { return 3i32 + helper() + fpi(); };