feat(tcl): add static variable name extraction and array key completion

This adds tooling to statically extract Tcl variable names from syntax
trees without evaluating substitutions, enabling better completions
for array elements and plain variables. The new helpers are wired
into the completion and navigation flows and are supported by tests
covering array keys and substitutions.

- Introduce variable_names.py with variable_name() and array_key_parts()
- Wire static name extraction into completion and symbol indexing
- Add tests for array key completion with substitutions

server/src/lsp_server.py

@@ -47,6 +47,7 @@ from common.load_data import standard_items
 from lsp_tclserver import TclLanguageServer
 from tools.completion_items import (
     CompletionContext,
+    array_element_completions,
     completion_context,
     ranked_completion_items,
 )
@@ -271,12 +272,18 @@ def document_diagnostic(params: lsp.DocumentDiagnosticParams):
 
 @LSP_SERVER.feature(
     lsp.TEXT_DOCUMENT_COMPLETION,
-    lsp.CompletionOptions(trigger_characters=["$", " ", "-"]),
+    lsp.CompletionOptions(trigger_characters=["$", " ", "-", "(", ","]),
 )
 def on_completion(params: lsp.CompletionParams) -> lsp.CompletionList:
     doc = LSP_SERVER.workspace.get_text_document(params.text_document.uri)
     position = params.position
     source_lines = LSP_SERVER.get_lines(doc)
+    array_items = array_element_completions(
+        source_lines, position, LSP_SERVER.navigation_snapshot().values(),
+        str(pathlib.Path(uris.to_fs_path(doc.uri))),
+    )
+    if array_items is not None:
+        return lsp.CompletionList(is_incomplete=False, items=array_items)
     context = completion_context(source_lines, position)
 
     # Variable completion wins inside command arguments. Otherwise prefer the
@@ -323,7 +330,7 @@ def on_completion(params: lsp.CompletionParams) -> lsp.CompletionList:
         and params.context is not None
         and params.context.trigger_kind
         == lsp.CompletionTriggerKind.TriggerCharacter
-        and params.context.trigger_character in {" ", "-"}
+        and params.context.trigger_character in {" ", "-", "(", ","}
     ):
         return lsp.CompletionList(is_incomplete=False, items=[])