feat(resolver): implement cross-file template resolution

Enable characters to inherit from templates defined in different files
across the project structure.

- Add file_index field to NameEntry to track declaration source files
- Update NameTable::from_files() to set file indices when merging tables
- Change conversion pipeline to pass &[ast::File] instead of flat arrays
- Update merge functions to use two-level indexing:
  all_files[entry.file_index].declarations[entry.decl_index]
- Update all affected tests to use new signatures

src/resolve/integration_tests.rs

@@ -123,7 +123,7 @@ fn test_all_declaration_kinds() {
             state s {}
         }
         schedule S {
-            10:00 -> 11:00: activity
+            10:00 -> 11:00: activity { }
         }
         behavior B {
             action