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/prop_tests.rs

@@ -52,6 +52,7 @@ fn valid_character_decl() -> impl Strategy<Value = (String, Declaration)> {
     valid_ident().prop_map(|name| {
         let decl = Declaration::Character(Character {
             name: name.clone(),
+            species: None,
             fields: vec![],
             template: None,
 
@@ -157,6 +158,7 @@ proptest! {
         let declarations: Vec<_> = (0..count).map(|i| {
             Declaration::Character(Character {
                 name: name.clone(),
+            species: None,
                 fields: vec![],
                 template: None,
 
@@ -177,6 +179,7 @@ proptest! {
             declarations: vec![
                 Declaration::Character(Character {
                     name: name.clone(),
+            species: None,
                     fields: vec![],
                     template: None,
 
@@ -254,6 +257,7 @@ proptest! {
             declarations: vec![
                 Declaration::Character(Character {
                     name: name.clone(),
+            species: None,
                     fields: vec![],
                     template: None,