feat(indexing): add persistent index cache and incremental reparse

- Pass extension storage path to the server (client/ changes) so the
  server can persist a workspace index.
- Introduce IndexCache (server/tools/index_cache.py) and load/save it on
  initialization and after background indexing. Index entries are stored
  only when the file's stat hasn't changed while being read.
- Add incremental reparse logic (server/tools/incremental_parse.py) and
  use a per-file _last_parse cache in the language server to reparse only
  the top-level Tcl commands touched by an edit, falling back to a full
  parse when necessary.
- Use a new _FileIndex dataclass and _build_file_index helper to unify
  what is stored/loaded for a file; update update_poco_completion_for_file
  to use the persistent cache for disk-read files (from_disk/source_stat).
- Keep background indexing non-blocking and persist the index at the
  end of the run. Add basic unit tests for incremental parse and index cache.

Before: edits and background work always required full parsing of files
and no persistent cross-restart index. After: some edits reuse previous
ASTs and files read from disk can use a persisted index to skip
re-indexing across restarts.

client/src/extension.ts

@@ -73,7 +73,13 @@ export async function activate(context: vscode.ExtensionContext) {
                 traceVerbose(
                     `Using interpreter from ${serverInfo.module}.interpreter: ${interpreter.join(" ")}`
                 )
-                client = await restartServer(serverId, serverName, outputChannel, client)
+                client = await restartServer(
+                    serverId,
+                    serverName,
+                    outputChannel,
+                    client,
+                    context.storageUri?.fsPath
+                )
             }
             return
         }
@@ -83,7 +89,13 @@ export async function activate(context: vscode.ExtensionContext) {
             traceVerbose(
                 `Using interpreter from Python extension: ${interpreterDetails.path.join(" ")}`
             )
-            client = await restartServer(serverId, serverName, outputChannel, client)
+            client = await restartServer(
+                serverId,
+                serverName,
+                outputChannel,
+                client,
+                context.storageUri?.fsPath
+            )
             return
         }