feat(imports): buildImportFacts raw-fact store (ModuleRawDeclIndex + NamespaceEdges) [stdlib A]

Phase A of the unified resolver (R5 locked design). Additive infrastructure
with NO behavior change — builds the import-side raw-fact store; nothing
consumes it yet.

- imports.zig: add RawDeclRef / RawAuthor / ModuleRawDeclIndex / ModuleDecls /
  NamespaceTarget / NamespaceEdges, plus buildImportFacts (mirrors
  buildModuleFns) producing a scalar per-module name→RawDeclRef index + the
  namespace edges. Callable without IR lowering (LSP reuses it later).
- ast.zig: NamespaceDecl gains target_module_path, captured at resolution time
  (the resolved_path otherwise lost on the node) so the namespace edge records
  the alias target.
- imports.zig: same-module duplicate top-level name is now DIAGNOSED
  ("duplicate top-level declaration 'X'") where addOwnDecl would silently drop
  the second author — replaces the discarded `_ =` at the three call sites.
- program_index.zig: borrowed views module_decls / namespace_edges (like
  module_fns); deinit does not free them.
- core.zig: build the facts alongside buildModuleFns and point the borrowed
  views at them.
- imports.test.zig: index unit tests (flat / directory / namespaced file /
  namespaced directory / C-import namespace / same-name fn / same-name struct /
  value-vs-type same spelling / raw const_decl) + the duplicate-name diagnostic
  regression (fails pre-fix, passes after).

Gate (worktree): zig build, zig build test (incl. LSP corpus sweep), and
run_examples (471, byte-identical) all green; m3te ios-sim build exits 0.

src/ir/program_index.zig

@@ -599,6 +599,14 @@ pub const ProgramIndex = struct {
     /// fix-0102c can resolve a flat call to the right module's function.
     /// Borrowed view.
     module_fns: ?*imports.ModuleFns = null,
+    /// Per-module scalar raw-decl index (`path → name → RawDeclRef`), built by
+    /// `imports.buildImportFacts`. The unified resolver's raw-fact store.
+    /// Borrowed view.
+    module_decls: ?*imports.ModuleDecls = null,
+    /// Namespace import edges (`importer → alias → NamespaceTarget`), built by
+    /// `imports.buildImportFacts`, carrying each alias's resolved target path.
+    /// Borrowed view.
+    namespace_edges: ?*imports.NamespaceEdges = null,
 
     // ── Declaration maps ──
     /// Function name → AST decl.
@@ -640,7 +648,7 @@ pub const ProgramIndex = struct {
 
     pub fn deinit(self: *ProgramIndex) void {
         // Owned maps only — module_scopes / import_graph / flat_import_graph /
-        // module_fns are borrowed.
+        // module_fns / module_decls / namespace_edges are borrowed.
         self.import_flags.deinit();
         self.fn_ast_map.deinit();
         self.qualified_fn_source.deinit();