fix(z-gap): close all 15 findings from xhigh-recall code review

[heznpc]

Critical fixes (paper claim integrity):

• V8: Strategy D/E/F sys.exit(2) on any failed model unless
  Z_GAP_ALLOW_PARTIAL_RESULTS=1. Holm-Bonferroni's "across 35 cells"
  claim can no longer be silently invalidated by a model dropout.
• V1/V14: SentenceTransformerEmbedder.name now includes full repo path
  (org/name → org__name) and @<sha8> revision suffix. C3 closure
  (PR experiments(z-gap): frozen model registry with HuggingFace SHAs (closes C3) #7) now actually holds end-to-end against SHA bumps and across
  org/name basename collisions.
• V2: run_cross_experiment_synthesis._normalize_results_envelope()
  unwraps {_meta, results} so legacy consumers keep working with the
  new D/E/F JSON shape; strategy_e and strategy_f added to known files.
• V3: paper §5.5 / Limitations "20 cells / four models / 20/20" drift
  updated to "35 cells / seven models / 35/35 + OOD 35/35" in 3 places
  (L463 method, L516 P2-resolution, L663 Limitations).

Statistical fixes (correctness):

• V5: compute_per_language_R_code substitutes NaN (not 1.0) when
  d_match_perm is empty or bootstrap mean_m ≤ 1e-10; np.nanmean for
  random_baseline_R aggregation. New null R range [1.0001, 1.0046]
  (tier1) / [1.0005, 1.0086] (OOD), unbiased by silent 1.0 imputations.
• V6: permutation p-value uses (k+1)/(n+1) convention. No cell reports
  literal 0.0 anymore (verified post-rerun: min nonzero p = 0.0001
  across all 70 D+F cells). Reviewer push-back surface closed.

Robustness / pitfall fixes:

• V7: Strategy D/E/F save JSON BEFORE generating figures; figures in
  try/except. Multi-hour compute no longer lost to matplotlib fail.
• V9: MistralEmbedder Retry sets respect_retry_after_header=False;
  bounded by backoff_factor=1 (~31s worst case), eliminating the
  multi-hour silent stall mode on server-sent Retry-After.
• V10: OpenAI client timeout 60s → 300s; legacy batch callers no longer
  regress on slow server-side processing.
• V11: Strategy E replaces categories[op_id] with categories.get() +
  _label() helper. Empty test sets produce {skip:true} cells with
  NaN accuracy instead of crashing on clf.predict(np.array([])).
• V12: load_ood_stimuli() asserts tier2/tier3 op_id uniqueness with the
  duplicate list in the error message. 50/50 unique today; future
  collision will fail loudly.
• V13: EmbeddingCache._key() switched from |-joined string to a
  JSON-encoded payload hash. ['a|b','c'] and ['a','b|c'] now hash
  to distinct keys.
• V18: SentenceTransformerEmbedder.dimension falls back to a single-text
  encode probe when the deprecated get_sentence_embedding_dimension()
  returns None. Nomic v1.5 no longer risks int(None) silent skip.
• V20: synthesis script counts aggregate as a 6th language → explicit
  if lang == "aggregate": continue in the per-language counter.

Hygiene:

• V4: Strategy D datetime.datetime.utcnow() → datetime.now(datetime.UTC),
  matching E/F and surviving future Python ≥3.13 removal.

Re-execution:

• D/E/F rerun successfully (7/7 models each, ~5 min wall time).
• 35/35 + multi-model P3 + 35/35 OOD all preserved.
• 2-decimal R_code values unchanged except UniXcoder tier1 (1.0649 ≈ 1.06,
  was printed 1.07).
• OOD Cohen's d_max E5-large 4.12 → E5-base 4.42; paper updated.

Decisions log:

• planning/decisions.md: 2026-05-21 entry covering all 15 fixes with
  per-finding rationale and the re-execution outcome.