refactor(imports): retain dup same-name fn authors + build identity indexes [0102a]

First of four fix-0102 sub-steps. Purely additive: retains data that the
flat/directory merge currently first-wins-drops and builds two identity
indexes for later bare-name disambiguation (fix-0102c). No resolution
change — the existing first-wins bare path still wins; suite unchanged.

- mergeFlat + directory merge: stop dropping a same-name FUNCTION authored
  by a different module/file. Non-function decls keep first-wins dedup; node
  identity dedup is untouched.
- flat_import_graph: a flat-only subset of import_graph, recording an edge
  only for a bare `#import` (imp.name == null), never a namespaced
  `ns :: #import`. Threaded through resolveImports/resolveDirectoryImport
  and into ProgramIndex.
- module_fns (path -> name -> *const FnDecl): per-module authored-function
  index mirroring module_scopes, built in core.zig from the main module +
  cache. Same-name cross-module authors stay distinct under their own paths.
- imports.test.zig: asserts both a.sx/b.sx greet authors are retained in
  module_fns and in the global flat list, and that flat_import_graph
  excludes the namespaced edge while import_graph includes it.

Gate (this worktree): zig build, zig build test (398/398),
bash tests/run_examples.sh (457 passed) all green.

src/ir/program_index.zig

@@ -1,5 +1,6 @@
 const std = @import("std");
 const ast = @import("../ast.zig");
+const imports = @import("../imports.zig");
 const types = @import("types.zig");
 const inst = @import("inst.zig");
 const errors = @import("../errors.zig");
@@ -570,9 +571,10 @@ pub const GlobalInfo = struct { id: inst.GlobalId, ty: TypeId };
 /// `self.program_index.<field>`; later phases hand collaborator modules a
 /// `*ProgramIndex` instead of `*Lowering`.
 ///
-/// OWNS the declaration maps below. BORROWS `module_scopes` / `import_graph`
-/// (pointers into maps owned by the compilation driver, `core.zig`) — those
-/// are read-only views and are never freed here.
+/// OWNS the declaration maps below. BORROWS `module_scopes` / `import_graph` /
+/// `flat_import_graph` / `module_fns` (pointers into maps owned by the
+/// compilation driver, `core.zig`) — those are read-only views and are never
+/// freed here.
 ///
 /// Per-map allocators are preserved exactly as they were on `Lowering`:
 /// `import_flags` / `fn_ast_map` / `global_names` use the lowering allocator
@@ -587,6 +589,16 @@ pub const ProgramIndex = struct {
     /// Module path → set of directly imported paths (param_impl visibility
     /// filter). Borrowed view.
     import_graph: ?*std.StringHashMap(std.StringHashMap(void)) = null,
+    /// Module path → set of directly FLAT-imported paths — the subset of
+    /// `import_graph` edges from a bare `#import` (never a namespaced
+    /// `ns :: #import`). fix-0102c's bare-name disambiguation walks this to
+    /// decide which same-name authors a flat importer can reach. Borrowed view.
+    flat_import_graph: ?*std.StringHashMap(std.StringHashMap(void)) = null,
+    /// Module path → (function name → authoring `*const FnDecl`), mirroring
+    /// `module_scopes`. Retains every same-name author under its own path so
+    /// fix-0102c can resolve a flat call to the right module's function.
+    /// Borrowed view.
+    module_fns: ?*imports.ModuleFns = null,
 
     // ── Declaration maps ──
     /// Function name → AST decl.
@@ -627,7 +639,8 @@ pub const ProgramIndex = struct {
     }
 
     pub fn deinit(self: *ProgramIndex) void {
-        // Owned maps only — module_scopes / import_graph are borrowed.
+        // Owned maps only — module_scopes / import_graph / flat_import_graph /
+        // module_fns are borrowed.
         self.import_flags.deinit();
         self.fn_ast_map.deinit();
         self.qualified_fn_source.deinit();