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/pos.ww

@@ -0,0 +1,18 @@
+// Positive case: import both modules, dispatch through the explicit
+// module qualifier. Each module's ping = bare-value + helper() (CALL
+// N_IDENT) + fpi() (which exercises LEAQ N_IDENT via `let h = helper`).
+//
+//   mod1.ping = 3 + 11 + 11 = 25
+//   mod2.ping = 5 + 13 + 13 = 31
+//   total     = 56
+//
+// Pre-fix the four `helper` / `ping` symbols would collapse at link
+// or the hint-less lookup would silently grab the wrong module's
+// body — exit would land at 50/62/some other value, never 56.
+
+use mod1;
+use mod2;
+
+fn main() i32 = {
+	return mod1.ping() + mod2.ping();
+};