feat(lower): source-aware forward-alias fixpoint [stdlib E1.5]

resolveForwardIdentifierAliases now resolves a forward alias A :: B against
B AS SEEN FROM A's own source via selectNominalLeaf (E1's source-keyed
nominal leaf over type_aliases_by_source / moduleTypeAuthor), never the
global type_alias_map / global findByName. The already-resolved guard is
per-source (aliasResolvedInSource). .pending routes back into the fixpoint;
.undeclared / .not_visible leave A unwritten (no global last-wins leak).

This is the sequencing pin before E2: a global fixpoint binds A to a
same-name B authored by a different module (e.g. a namespaced import that
pollutes the global alias map last-wins), re-opening 0105 one layer down
once shadows register. Writes stay on the unified putTypeAlias helper (E1
no-drift invariant); the single graph-walk in resolver.zig is untouched.

Regression: examples/0750-modules-forward-alias-source-aware — a forward
alias A :: B with main's own B :: u64 and a namespaced same-name B :: u8;
A must bind main's u64 (300), not the global last-wins u8 (44).

examples/0750-modules-forward-alias-source-aware.sx

@@ -0,0 +1,31 @@
+// Source-aware forward-alias fixpoint (R5 §4, E1.5). A forward identifier alias
+// `A :: B` must resolve its target `B` AS SEEN FROM ITS OWN SOURCE, not via the
+// global `type_alias_map` (which is last-wins across every module).
+//
+// `main` authors a forward alias `A :: B` and its own `B :: u64`. The namespaced
+// import `ns :: #import ".../dep.sx"` ALSO authors a top-level `B :: u8`; being
+// scanned after main's `B`, dep's alias is what the GLOBAL `type_alias_map["B"]`
+// ends up holding (last-wins). A global forward-alias fixpoint therefore bound
+// `A` to dep's `u8` — re-opening 0105 one layer down. The source-aware fixpoint
+// resolves `A`'s target against MAIN's source, binding the local `B :: u64`.
+//
+// Observable: a runtime 300 coerced into an `A`-typed slot round-trips as 300
+// when `A` is `u64` (correct) and truncates to 44 when `A` is wrongly `u8`.
+// The direct reference `b : B` already resolves source-aware via E1's nominal
+// leaf, so it pins the same `u64` for contrast. Regression (stdlib E1.5).
+#import "modules/std.sx";
+
+A :: B;
+B :: u64;
+
+ns :: #import "0750-modules-forward-alias-source-aware/dep.sx";
+
+main :: () -> s32 {
+    n : s64 = 300;
+    a : A = xx n;
+    b : B = xx n;
+    print("forward A (u64=300): {}\n", cast(s64) a);
+    print("direct  B (u64=300): {}\n", cast(s64) b);
+    print("ns.width():          {}\n", ns.width());
+    return 0;
+}

examples/0750-modules-forward-alias-source-aware/dep.sx

@@ -0,0 +1,10 @@
+// Namespaced helper module. It authors a top-level type alias `B` whose
+// spelling collides with the importer's own `B`. Because the import is
+// NAMESPACED (`ns :: #import`), `dep.B` is NOT flat-visible to the importer —
+// but its alias write still lands in the global `type_alias_map` (last-wins),
+// which is exactly what the source-aware forward-alias fixpoint must ignore.
+B :: u8;
+
+width :: () -> s32 {
+    return 8;
+}

examples/expected/0750-modules-forward-alias-source-aware.exit

@@ -0,0 +1 @@
+0

examples/expected/0750-modules-forward-alias-source-aware.stderr

@@ -0,0 +1 @@
+

examples/expected/0750-modules-forward-alias-source-aware.stdout

@@ -0,0 +1,3 @@
+forward A (u64=300): 300
+direct  B (u64=300): 300
+ns.width():          8