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/common/server.ts

@@ -22,7 +22,12 @@ import {
 import { getLSClientTraceLevel, getProjectRoot } from "./utilities"
 import { isVirtualWorkspace } from "./vscodeapi"
 
-export type IInitOptions = { settings: ISettings[]; globalSettings: ISettings }
+export type IInitOptions = {
+    settings: ISettings[]
+    globalSettings: ISettings
+    // Folder for the server's persistent index cache; omitted without a workspace.
+    indexCachePath?: string
+}
 
 let _disposables: Disposable[] = []
 
@@ -114,7 +119,8 @@ export async function restartServer(
     serverId: string,
     serverName: string,
     outputChannel: LogOutputChannel,
-    lsClient?: LanguageClient
+    lsClient?: LanguageClient,
+    indexCachePath?: string
 ): Promise<LanguageClient | undefined> {
     if (lsClient) {
         traceInfo(`Server: Stop requested`)
@@ -132,7 +138,8 @@ export async function restartServer(
             outputChannel,
             {
                 settings: await getExtensionSettings(serverId, true),
-                globalSettings: await getGlobalSettings(serverId, false)
+                globalSettings: await getGlobalSettings(serverId, false),
+                indexCachePath
             }
         )
         traceInfo(`Server: Start requested.`)