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.

cmd/w6c/gc.h

@@ -34,6 +34,13 @@ struct Cg {
 	Arena	*a;
 	Prog	*head, *tail;
 	const char *fnname;
+	const char *cur_mod;	/* current fn's `// MODULE: foo` directive,
+				 * NULL when the fn lives in the primary
+				 * file (no MODULE: stamp). Drives bare-IDENT
+				 * call mangling — `frob()` from within
+				 * lib/foo binds to `foo.frob` regardless
+				 * of which other modules also export `frob`.
+				 * Set by cgfn before walking the body. */
 	int	framesize;	/* bytes of locals; 16-byte aligned */
 	int	curoff;		/* current top of locals */
 	Scope	*locals;	/* (name → offset) tracked via Sym */