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/links_prop_tests.rs

@@ -458,9 +458,9 @@ proptest! {
             prop_oneof![
                 valid_ident().prop_map(|name| Declaration::Character(Character {
                     name,
+                    species: None,
                     fields: vec![],
                     template: None,
-
                     span: Span::new(0, 10),
                 })),
                 valid_ident().prop_map(|name| Declaration::Template(Template {