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/tools/semantic_tokens.py

@@ -5,6 +5,7 @@ import attrs
 from common.load_data import standard_items
 from tclint.commands.plugins import PluginManager
 from tclint.syntax_tree import BareWord, Command, QuotedWord, Visitor
+from tools.variable_names import variable_name
 
 # Constructing a PluginManager scans entry points, and get_commands() rebuilds
 # the builtin command set on every call. Semantic tokens are requested often, so
@@ -159,6 +160,11 @@ class _Highlighter(Visitor):
         if routine.contents == "set" and command.args:
             first_arg = command.args[0]
             token_info = self._get_token_info(first_arg)
+            if first_arg.contents is None:
+                name = variable_name(first_arg)
+                if name:
+                    line, col = first_arg.children[0].pos
+                    token_info = ((line - 1, col - 1), len(name))
             if token_info:
                 (line, col), length = token_info
                 self._append_token((line, col), length, "variable", [TokenModifier.declaration])