wwstage: make the cgdot alias-peel struct-break module-aware (#223)

The wwstage cgdot #191 alias-peel loop broke on a name-keyed any-module
structlookup, so a receiver whose alias name collides with a struct of the
same name in ANOTHER module resolved to the foreign struct and fell through to
an undefined `name(SB)` global instead of the field load. The eFinal FLIP
renames io's `vstream` -> `stream`, which collides with memio's `stream`
struct, so io.read/io.write/io.close's `match (s.reader)` emitted
`MOVQ reader(SB), AX` (reader is also a type-alias) -> cs != ww (cstage chases
the nominal TY_NAMED.under pointer chain, module-correct). Gate-blind: on
master both stages emit the same wrong store so byte-id stays green; the FLIP
corpus is the first to put the io-alias/memio-struct collision in one build.

Fix (wwstage-only align-down; cstage is the authority and is untouched): make
the peel's struct-break MODULE-AWARE — break only on a same-module struct (a
genuine struct-value receiver); a same-module alias keeps peeling to its
underlying (io.stream -> *vtable -> the pointer field-load arm); a foreign leaf
keeps the prior any-module heuristic. New structsamemod / aliassamemod mirror
the same-module-first pass already in structlookup / aliaslookup. This is not a
naive alias-first reorder (which would reintroduce the mirror collision: a
same-module struct plus a foreign same-leaf alias). Peel-only — the direct-
struct arm's broader cross-module same-leaf-STRUCT name-keying is filed as #224.

#208-family (name-keyed resolution dropping to a wrong global) but in cgen, not
the checker; #213 is distinct (cosmetic local-struct-match divergence).

test/wcc/784_xmod_alias_struct_collide_run: collision (cross-module alias-vs-
struct, same leaf), symmetric (guards the same-module-struct break against a
naive reorder), and a no-collision control — branched callee. The discriminating
net is cs.s == ww.s (the path is gate-blind and cstage is correct, so byte-id
flips when wwstage is fixed); confirmed by source-revert. The FLIP's combined.ww
is now cs.s == ww.s byte-identical.

selfhost/cmd/wcc/cgenutil.ww

@@ -1381,6 +1381,23 @@ fn structlookup(c: *cgen, name: str) *structinfo = {
 	return nil;
 };
 
+// #223: same-module-ONLY struct lookup. structlookup's any-module
+// fallback returns a foreign same-leaf struct; the cgdot alias-peel
+// needs to break ONLY on a struct that THIS module defines (a genuine
+// struct-value receiver), not on a foreign struct that merely shares a
+// leaf with a same-module alias (io.stream alias vs memio.stream
+// struct). Returns the struct only when it lives in c.curmod.
+fn structsamemod(c: *cgen, name: str) *structinfo = {
+	let s: *structinfo = c.structs;
+	for (s != nil) {
+		if (streq(s.sname, name)) {
+			if (streq(s.smod, c.curmod)) { return s; };
+		};
+		s = s.sinext;
+	};
+	return nil;
+};
+
 // primsize — size in bytes of a primitive type name (or 0 if not
 // recognised as a primitive — the caller falls back to other paths).
 // fldnumidx — parse a tuple field name like "0" / "1" / "12" into an