fix: resolve qualified-import-member const as a compile-time constant (issue 0192)

A namespaced import's const (`m :: #import "lib.sx"; … m.CAP`) only ever
resolved as a runtime value — the const folders in program_index.zig had no
namespace-member arm, so a qualified const was rejected as an array dimension /
Vector lane / generic value-param and could not seed another const, while the
flat-import form worked everywhere.

Add a `lookupQualifiedConst` (+ float / float-typed twins) ctx hook: resolve
the alias via `namespaceAliasVerdictFrom` to its target module, then fold the
member from that module's per-source const cache (`foldQualifiedConstInt` in
lower/comptime.zig), pinned to the target source so nested const RHSs fold
there. Wire it into evalConstIntExpr / evalConstFloatExpr / isFloatValuedExpr —
both the expression-position field_access arm (`[m.CAP]T`) and the
type-argument dotted-name arm (`Vector(m.LANES, …)`, generic value-params).

Implemented on the source-aware ctxs (Lowering / SourceConstCtx); the
namespace-blind ModuleConstCtx / StatelessInner return null, so a qualified-const
dim reached only via the stateless type-alias path stays a clean unresolved-dim
diagnostic, never a fabricated length. Resolves correctly for array dims,
arithmetic, integral-float dims, Vector lanes, generic value-params, inline-for
bounds, and struct fields.

Regression: examples/modules/0842-modules-qualified-import-const-comptime.sx.

src/ir/program_index.test.zig

@@ -188,6 +188,17 @@ const DimCtx = struct {
     pub fn nameIsFloatTyped(_: DimCtx, name: []const u8) bool {
         return std.mem.eql(u8, name, "F") or std.mem.eql(u8, name, "K");
     }
+    // This test ctx models no namespace imports — qualified-member consts
+    // (`m.CAP`, issue 0192) are exercised end-to-end by the corpus, not here.
+    pub fn lookupQualifiedConst(_: DimCtx, _: []const u8, _: []const u8) ?i64 {
+        return null;
+    }
+    pub fn lookupQualifiedConstFloat(_: DimCtx, _: []const u8, _: []const u8) ?f64 {
+        return null;
+    }
+    pub fn qualifiedNameIsFloatTyped(_: DimCtx, _: []const u8, _: []const u8) bool {
+        return false;
+    }
 };
 
 fn nLit(v: i64) ast.Node {