Add librarian Stage 1 citation checker with ignore list

scripts/librarian_check_citations.py extracts file:line references from
inline-code spans across docs/**/*.md and verifies each path exists
(and, if cited with a line number, is within range). Skips
docs/archive/ and docs/plans/archive/ which are read-only by policy.

Caught one real drift in docs/research/optimization_sequencing.md: the
note pointed at docs/plans/amp_trainer.md, which had moved into
docs/plans/archive/.

scripts/librarian-ignore.txt holds fnmatch globs for citations that are
intentionally future-tense (planned files described in the plan docs
themselves). Used sparingly so the checker stays useful as a drift
signal.

Updates docs/plans/librarian.md Progress + Next Step. Next concrete
step is a thin scripts/librarian.sh orchestrator over both checkers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/research/optimization_sequencing.md

@@ -8,7 +8,7 @@
 | --- | --- | --- |
 | **Model size growth** (hidden ≥ 1024 / layers ≥ 6) | `docs/plans/model_size_experiment.md` | 인프라 미설치 |
 | **Option B** (per-worker interleaved traversal) | plan 미작성 | 미시작 |
-| **AMP** trainer | `docs/plans/amp_trainer.md` (구현 됨, default off) | 모델 키운 후 재측정 |
+| **AMP** trainer | `docs/plans/archive/amp_trainer.md` (구현 됨, default off) | 모델 키운 후 재측정 |
 | **torch.compile** trainer | `docs/plans/torch_compile.md` | 모델 키운 후 재측정 |
 | **TensorRT** inference | plan 미작성 | 모델 키운 + eval dense 시점 |
 | **Option A re-enable** | 코드 있음 (default off) | 모델 키운 후 또는 Option B 후 |