selfhost+cstage+test: graduate enumlookup same-module-first + N_DOT enumlookupmod (#4a)

Class A silent miscompile, latent until two modules export the same
enum leaf name. Wwstage's enumlookup (selfhost/cmd/wcc/cgen.ww)
walked c.enums head-first by ename; cgdot handed it the bare leaf
from N_DOT.lhs.str for both `Color.MEMBER` (lhs N_IDENT) and
`pkg.Color.MEMBER` (lhs N_DOT) shapes, silently dropping the
explicit qualifier on the second. Cstage's enum-member fold
(cmd/wcc/check.c cexpr N_DOT) was carrying the same head-pick on
the lhs-ident lookup — pre-fix the mismatch surfaced as a
"not assignable to <same-leaf>" checker error rather than a silent
wrong-constant because resolve_typename for the fn return spec
already used scope_lookup_prefer correctly, so the rhs's wrong-
module-Color clashed with the return type's right-module-Color.
No in-tree corpus currently declares two same-leaf enums, so
995_self_rebuild stayed green and the latent miscompile only
surfaces once a stdlib port introduces the collision (same shape
as #27 surfacing when lib/strings dragged utf8's invalid alias
into the chain alongside strconv's invalid).

Fourth leaf of the trio leaf-name lookup graduation (after #27
aliaslookup, #28 fnparamslookupmod, #31 fnretlookupmod): wwstage
enumlookup grows a same-module-first walk before the head-walk
fallback, mirroring aliaslookup's two-pass shape (cgen.ww:75).
The N_DOT consumer surface — `pkg.Enum.MEMBER`, already used
in-corpus by os.flag.RDONLY, temp.mode.RDWR, os.whence.SET etc.
— routes through a new enumlookupmod variant with the explicit
N_DOT.lhs.lhs.str as the mod qualifier (mirror of fnret/
fnparamslookupmod). Cstage's check.c cexpr N_DOT lhs lookup
graduates from scope_lookup to scope_lookup_prefer to align
symmetrically (rule 10: both stages pick same-module-first on
the bare-leaf shape).

733_enum_modshadow pins both surfaces with 3 rows: row 1 bare-leaf
in module M must fold against M's own Color even with another
module's same-leaf Color at the head of c.enums; row 2 same-module
`mod.Color.MEMBER` from inside that mod pins the API surface; row 3
cross-module `othermod.Color.MEMBER` from a third module with no
local Color sentinel-flips the cgdot etmod tracking + enumlookupmod
path independently of row 1's same-module-first fallback. Asserts
the matching \$N, immediate inside the right TEXT sym + bad_imm
NOT-presence anti-check on both stages plus byte-id between stages
per row.

cmd/wcc/check.c

@@ -737,9 +737,14 @@ cexpr(Checker *c, Node *n)
 		/* module-qualified: lhs is an N_IDENT bound as SK_USE.
 		 * Resolve to the symbol with the same leaf name. With
 		 * driver-side concatenation, all symbols live in flat
-		 * scope, so we lookup `n->str` directly. */
+		 * scope, so we lookup `n->str` directly. Same-module-
+		 * first via _prefer keeps a bare-leaf enum `Color.M`
+		 * inside module M from collapsing onto another module's
+		 * Color sitting at the head of the flat scope chain —
+		 * symmetric with wwstage's enumlookup graduation. */
 		if (n->lhs && n->lhs->kind == N_IDENT) {
-			Sym *ms = scope_lookup(c->cur, n->lhs->str);
+			Sym *ms = scope_lookup_prefer(c->cur, c->cur_mod,
+			    n->lhs->str);
 			if (ms && (ms->kind == SK_USE || ms->use_alias)) {
 				/* Module-qualified ref. `use_alias` covers
 				 * the self-import case where the module's