feat(loop): complete Issue #5 Phase 4 immutable runtime and freeze removal

- Implement eager module loading (lib_py.loop.eager) to preload all framework
  dependencies into sys.modules and enforce PYTHONDONTWRITEBYTECODE=1.
- Synchronize runtime protocol version with MAM_VERSION constant (4.3.0).
- Bypass /tmp/mam-loop-freeze.XXXXXX copy overhead by default in Python mode
  while preserving legacy rollback fallback (MAM_LOOP_PY=0 / MAM_LOOP_FREEZE=1).
- Add tests/test_issue5_phase4_runtime.py verifying eager import completeness,
  mid-flight code modification immunity (B-13), and freeze bypass.
- Pass full test suite (749 tests, 0 failures).

.agents/skills/lib_py/loop/runner.py

@@ -8,6 +8,7 @@ while using fence-aware strict verdict parsing and atomic checkpointing.
 from __future__ import annotations
 
 import argparse
+import atexit
 import glob
 import json
 import os
@@ -21,7 +22,7 @@ import time
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Tuple
 
-from lib_py.loop import checkpoint, parser
+from lib_py.loop import checkpoint, eager, parser
 
 # ANSI colors matching run_loop.sh
 _COLOR_INFO = "\033[1;34m[*]\033[0m"
@@ -50,6 +51,9 @@ class LoopRunner:
     """Orchestrates planning, code implementation, and multi-agent peer reviews."""
 
     def __init__(self, argv: Optional[List[str]] = None):
+        # Eagerly load all required framework modules to ensure immutable runtime (Phase 4)
+        eager.eager_load()
+
         self.raw_argv = list(argv) if argv is not None else sys.argv[1:]
         self.workspace_root = Path(os.environ.get("MAM_REAL_ROOT") or os.environ.get("WORKSPACE_ROOT") or Path.cwd()).resolve()
         self.mam_dir = self.workspace_root / ".mam"
@@ -372,7 +376,6 @@ class LoopRunner:
     def run(self) -> int:
         self.parse_args()
 
-        import atexit
         atexit.register(self.release_guard)
 
         # Signal handling