cstage+test: SK_USE→SK_X promotion sets use_alias

In cmd/wcc/check.c the pass-1.5 SK_USE→SK_DEF/SK_FN/SK_VAR promotion
sites forgot to set prev->use_alias = 1 when the imported module's
top-level decl shadowed the SK_USE leaf in flat scope. Downstream
dot-prefixed lookups (resolve_typename L77, N_DOT L709) gate the
module-head walk on (SK_USE || use_alias), so `mod.flag` resolution
fell through to "unknown type". The SK_TYPE precedent at L1660 had
the line; the three sister sites at L1709/L1722/L1736 now do too,
in the same one-line shape and field-set order.

The wwstage selfhost/cmd/wcc/check.ww uses coexistence rather than
in-place promotion: SK_USE and same-leaf SK_TYPE/FN/DEF/VAR live as
separate entries differentiated by sym.mod, and scopelookupinmodule's
mod-filter already disambiguates dotted lookups — no use_alias flag
needed, so the cstage bug is structurally non-reachable there. An
architectural note at installdecl documents this divergence-by-design
and warns against porting the flag (adding a field to `sym` changes
its size and risks the wwstage cgen amalloc-undersize trap).

Audit covered every SK_USE→SK_X promotion path in check.c (4 sites:
SK_TYPE already-correct as precedent, SK_DEF/SK_FN/SK_VAR fixed). The
surfacing case was lib/fnmatch: `fn fnmatch(...)` shadows the SK_USE
leaf, so `fnmatch.flag` failed in worker-fnmatch's WIP — that test
(972_fnmatch_run) now flips PASS as live integration proof.
test/wcc/699_use_promote_alias.c pins all four rows with a single
table-driven driver (type/fn/def/var → use mod; let m: mod.flag =
mod.flag.A; return m: i32, expecting exit 42 per row). 995_self_rebuild
byte-identity holds.

test/wcc/data/usepromote/defmod/defmod.ww

@@ -0,0 +1,13 @@
+// usepromote/defmod — exports a `def defmod` with the same leaf as
+// the module name. Paired with pos_def.ww to exercise the SK_USE→
+// SK_DEF promotion (cmd/wcc/check.c L1709). Mirrors the SK_FN bug
+// shape: without `use_alias = 1`, the consumer's `defmod.flag` lookup
+// fails because the promoted-in-place SK_DEF leaf no longer advertises
+// itself as a module head.
+
+export def defmod: i32 = 0i32;
+
+export type flag = enum i32 {
+	NONE = 0,
+	A    = 42,
+};

test/wcc/data/usepromote/fnmod/fnmod.ww

@@ -0,0 +1,17 @@
+// usepromote/fnmod — exports a `fn fnmod()` with the same leaf as the
+// module name. Paired with pos_fn.ww to exercise the bug fix at
+// cmd/wcc/check.c L1722 (SK_USE→SK_FN promotion was missing the
+// `use_alias = 1` line, so the consumer's `fnmod.flag` lookup walked
+// resolve_typename's dot branch, found head `fnmod` as SK_FN, and
+// skipped the module-filtered leaf path → "unknown type fnmod.flag".
+//
+// lib/fnmatch is the real-world instance that surfaced this.
+
+export type flag = enum i32 {
+	NONE = 0,
+	A    = 42,
+};
+
+export fn fnmod() i32 = {
+	return 0i32;
+};

test/wcc/data/usepromote/pos_def.ww

@@ -0,0 +1,12 @@
+// SK_USE→SK_DEF promotion. Same shape as pos_fn.ww but the imported
+// module exports `def defmod: i32 = 0` instead of `fn defmod()`.
+// Pass-1.5 N_DEF case (check.c L1709) promotes the SK_USE leaf to
+// SK_DEF. Pre-fix it forgot use_alias=1, so `defmod.flag` resolution
+// failed. Post-fix the build succeeds and exit code = flag.A = 42.
+
+use defmod;
+
+fn main() i32 = {
+	let m: defmod.flag = defmod.flag.A;
+	return m: i32;
+};

test/wcc/data/usepromote/pos_fn.ww

@@ -0,0 +1,15 @@
+// SK_USE→SK_FN promotion (the bug). fnmod exports `fn fnmod()` and
+// `type flag = enum`. Pass-1 installs SK_TYPE("flag", mod="fnmod") and
+// SK_USE("fnmod") (the `fn fnmod` isn't installed in pass-1; pass-1
+// only handles N_USE + N_TYPEDECL). Pass-1.5 then sees the N_FNDECL
+// "fnmod", finds the SK_USE local, and promotes in place to SK_FN.
+// Pre-fix that promotion forgot `use_alias = 1`, so `fnmod.flag`
+// resolution failed with "unknown type fnmod.flag". Post-fix the
+// build succeeds and exit code = flag.A = 42.
+
+use fnmod;
+
+fn main() i32 = {
+	let m: fnmod.flag = fnmod.flag.A;
+	return m: i32;
+};

test/wcc/data/usepromote/pos_type.ww

@@ -0,0 +1,14 @@
+// SK_USE→SK_TYPE regression pin. typmod exports `type typmod = ...`
+// AND `type flag = enum`. Driver concatenation puts imported decls
+// first, so pass-1 installs SK_TYPE("typmod") first; the primary's
+// `use typmod` then hits the self-import branch (check.c L1660) and
+// sets use_alias=1 on the SK_TYPE entry. We rely on use_alias=1 here
+// so the dot-prefixed `typmod.flag` lookup resolves to mod="typmod"'s
+// flag entry. Exit code = flag.A = 42 verifies end-to-end.
+
+use typmod;
+
+fn main() i32 = {
+	let m: typmod.flag = typmod.flag.A;
+	return m: i32;
+};

test/wcc/data/usepromote/pos_var.ww

@@ -0,0 +1,12 @@
+// SK_USE→SK_VAR promotion. Imported module exports a top-level
+// `let varmod: i32 = 0`. Pass-1.5 N_LET case (check.c L1736) promotes
+// the SK_USE leaf to SK_VAR. Pre-fix it forgot use_alias=1, so
+// `varmod.flag` resolution failed. Post-fix the build succeeds and
+// exit code = flag.A = 42.
+
+use varmod;
+
+fn main() i32 = {
+	let m: varmod.flag = varmod.flag.A;
+	return m: i32;
+};

test/wcc/data/usepromote/typmod/typmod.ww

@@ -0,0 +1,17 @@
+// usepromote/typmod — exports a `type typmod` with the same leaf as
+// the module name. Paired with pos_type.ww to pin the SK_USE→SK_TYPE
+// promotion's use_alias=1 behaviour (cmd/wcc/check.c L1660 / L1677):
+// the consumer's `typmod.flag` lookup must walk through the alias
+// even though the leaf sym is SK_TYPE, not SK_USE.
+//
+// random.random in lib/ is the canonical real-world instance of this
+// shape; this fixture replays it as a regression pin.
+
+export type typmod = struct {
+	x: i32,
+};
+
+export type flag = enum i32 {
+	NONE = 0,
+	A    = 42,
+};

test/wcc/data/usepromote/varmod/varmod.ww

@@ -0,0 +1,12 @@
+// usepromote/varmod — exports a top-level `let varmod` with the same
+// leaf as the module name. Paired with pos_var.ww to exercise the
+// SK_USE→SK_VAR promotion at cmd/wcc/check.c L1736. Same shape as
+// SK_DEF / SK_FN — without `use_alias = 1`, the consumer's
+// `varmod.flag` lookup fails.
+
+export let varmod: i32 = 0i32;
+
+export type flag = enum i32 {
+	NONE = 0,
+	A    = 42,
+};