fix(ir): resolve forward identifier type aliases in scanDecls (issue 0069)

scanDecls' `.identifier` alias branch registered `A :: B` into
ProgramIndex.type_alias_map only when `B` was already known (in
type_alias_map or the TypeTable). A forward target declared later
(`MyChain :: MyInt; MyInt :: s32;`) was never present during the single
forward scan, so the alias name went unregistered and the A2.4
unknown-type pass — which treats type_alias_map keys as declared types —
flagged its uses as `unknown type 'MyChain'`.

Add a fixpoint post-pass `resolveForwardIdentifierAliases` at the end of
scanDecls that re-resolves identifier-RHS aliases until no progress, after
every top-level name has been seen. A value const is never an `.identifier`
node, and an alias whose target is a value const still misses both lookups,
so issue 0068's value-const rejection is preserved.

Regression: examples/0132-types-forward-type-alias.sx (forward alias +
forward chain). Gate: zig build, zig build test, run_examples.sh -> 353/0.

examples/0132-types-forward-type-alias.sx

@@ -0,0 +1,24 @@
+// Forward identifier type alias — an alias whose target is declared LATER
+// in the file resolves the same as an ordered one. `MyChain :: MyInt;`
+// appears before `MyInt :: s32;`, yet `MyChain` resolves to `s32` and a
+// forward chain (`A :: B; B :: C; C :: u8;`) converges too.
+// Regression (issue 0069): the scan only registered identifier aliases whose
+// target was already known, so a forward alias was falsely flagged
+// `unknown type`. Now a fixpoint pass over the scanned decls resolves them.
+#import "modules/std.sx";
+
+MyChain :: MyInt;
+MyInt   :: s32;
+
+A :: B;
+B :: C;
+C :: u8;
+
+main :: () -> s32 {
+    v: MyChain = 7;
+    n: A = 3;
+    print("chain s32:   {}\n", size_of(MyChain));
+    print("forward u8:  {}\n", size_of(A));
+    print("v + n:       {}\n", v + cast(s32) n);
+    return v;
+}

examples/expected/0132-types-forward-type-alias.exit

@@ -0,0 +1 @@
+7

examples/expected/0132-types-forward-type-alias.stderr

@@ -0,0 +1 @@
+

examples/expected/0132-types-forward-type-alias.stdout

@@ -0,0 +1,3 @@
+chain s32:   4
+forward u8:  1
+v + n:       10