feat(tree-sitter-overlay-extra): parse JS/C/Java overlay via optional extra

Add openultrasast[semantic] with official grammar wheels and one CST walker
into FileIR. quick stays extra-free. Host CLI is not overlay-ready. Default CI
stays extra-free; a semantic job runs pytest -m semantic.

src/openultrasast/semantic/extra.py

@@ -0,0 +1,48 @@
+"""Optional semantic extra: lazy grammar load. Never import wheels at package load."""
+
+from __future__ import annotations
+
+import importlib
+import importlib.util
+import os
+from typing import Any
+
+_GRAMMAR_MODULES = {
+    "python": "tree_sitter_python",
+    "javascript": "tree_sitter_javascript",
+    "c": "tree_sitter_c",
+    "cpp": "tree_sitter_cpp",
+    "java": "tree_sitter_java",
+}
+_PROBE_ENV = "OPENULTRASAST_TREE_SITTER_PROBE"
+_PROBE_OFF = frozenset({"0", "off", "false", "unavailable", "no"})
+_PROBE_ON = frozenset({"1", "on", "true", "available", "yes"})
+
+
+def has_semantic_extra() -> bool:
+    """True when the parser runtime and at least one in-scope grammar are importable."""
+    flag = os.environ.get(_PROBE_ENV, "").strip().lower()
+    if flag in _PROBE_OFF:
+        return False
+    if importlib.util.find_spec("tree_sitter") is None:
+        return False
+    return any(importlib.util.find_spec(module) is not None for module in _GRAMMAR_MODULES.values())
+
+
+def grammar_for(language: str) -> Any | None:
+    """Return a tree_sitter.Language for an in-scope language, or None. Never raises into the scan."""
+    flag = os.environ.get(_PROBE_ENV, "").strip().lower()
+    if flag in _PROBE_OFF:
+        return None
+    module_name = _GRAMMAR_MODULES.get(language)
+    if module_name is None or importlib.util.find_spec("tree_sitter") is None:
+        return None
+    if importlib.util.find_spec(module_name) is None:
+        return None
+    try:
+        from tree_sitter import Language
+
+        module = importlib.import_module(module_name)
+        return Language(module.language())
+    except Exception:
+        return None