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.

server/src/tools/navigation.py

@@ -265,6 +265,12 @@ def build_file_symbol_index(
     filepath: str, uri: str, tree: Node
 ) -> FileSymbolIndex:
     occurrences: list[SymbolOccurrence] = []
+    # Most occurrences repeat a few identities; sharing one object per identity
+    # keeps the index (and its persistent cache) small.
+    identities: dict[SymbolIdentity, SymbolIdentity] = {}
+
+    def shared(identity: SymbolIdentity | None) -> SymbolIdentity | None:
+        return None if identity is None else identities.setdefault(identity, identity)
 
     def add_proc(
         node: Node,
@@ -274,7 +280,7 @@ def build_file_symbol_index(
         is_definition: bool,
         declaration_range: lsp.Range | None = None,
     ) -> None:
-        identity = _proc_identity(raw_name, scope.namespace)
+        identity = shared(_proc_identity(raw_name, scope.namespace))
         caller = None
         if not is_definition:
             caller = (
@@ -288,14 +294,14 @@ def build_file_symbol_index(
                 fallback_identity=(
                     None
                     if is_definition
-                    else _proc_fallback(raw_name, scope.namespace)
+                    else shared(_proc_fallback(raw_name, scope.namespace))
                 ),
                 range=_name_range(node, raw_name),
                 placeholder=_basename(raw_name),
                 is_definition=is_definition,
                 symbol_kind=lsp.SymbolKind.Function,
                 container_name=_container_name(identity),
-                caller=caller,
+                caller=shared(caller),
                 declaration_range=declaration_range,
             )
         )
@@ -309,7 +315,7 @@ def build_file_symbol_index(
         variable_sub: bool = False,
         identity: SymbolIdentity | None = None,
     ) -> None:
-        symbol_identity = identity or _variable_identity(raw_name, scope)
+        symbol_identity = shared(identity or _variable_identity(raw_name, scope))
         occurrences.append(
             SymbolOccurrence(
                 identity=symbol_identity,