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/root.zig

@@ -13,6 +13,7 @@ pub const trace_runtime_tests = @import("runtime_trace.test.zig");
 pub const sema = @import("sema.zig");
 pub const sema_tests = @import("sema.test.zig");
 pub const imports = @import("imports.zig");
+pub const imports_tests = @import("imports.test.zig");
 pub const core = @import("core.zig");
 pub const c_import = @import("c_import.zig");
 pub const ir = @import("ir/ir.zig");