feat(decoration)!: lazy wrapping — import torchlens has no side effects

BREAKING CHANGE: torch functions are no longer wrapped at import time.
Wrapping happens lazily on first log_forward_pass() call and persists.

Three changes:

1. Lazy decoration: removed decorate_all_once() / patch_detached_references()
   calls from __init__.py. _ensure_model_prepared() triggers wrapping on
   first use via wrap_torch().

2. Public wrap/unwrap API:
   - torchlens.wrap_torch() — install wrappers (idempotent)
   - torchlens.unwrap_torch() — restore original torch callables
   - torchlens.wrapped() — context manager (wrap on enter, unwrap on exit)
   - log_forward_pass(unwrap_when_done=True) — one-shot convenience
   Old names (undecorate_all_globally, redecorate_all_globally) kept as
   internal aliases.

3. torch.identity fix: decorated identity function now stored on
   _state._decorated_identity instead of monkey-patching torch.identity
   (which doesn't exist in PyTorch type stubs). Eliminates 2 mypy errors.

Tests updated: 75 pass including 12 new lifecycle tests.

Author: johnmarktaylor91