feat(lsp): add context-aware completion ranking and document highlights

Adds context-aware completion ranking and document highlights for Tcl.
Adds completion_context to distinguish variables and commands.
Wires per-file completion snapshots and ranking into the flow.
Adds document_highlight provider support and tests for highlights.

- Context-aware ranking of completion items using per-file snapshots
- Document highlight provider wired into initialization and tests
- Tests for completion context, ranking, and document highlights

server/src/lsp_tclserver.py

@@ -170,6 +170,15 @@ class TclLanguageServer(LanguageServer):
             self._workspace_completion_cache = (self._index_generation, items)
             return items
 
+    def completion_items_by_file_snapshot(
+        self,
+    ) -> dict[str, tuple[lsp.CompletionItem, ...]]:
+        """Return completion items grouped by source file for request ranking."""
+        with self._index_lock:
+            return {
+                path: tuple(items) for path, items in self.poco_completion.items()
+            }
+
     def custom_function_names_snapshot(self) -> frozenset[str]:
         """Return custom completion labels for semantic highlighting."""
         with self._index_lock: