distillation: fix 7 grounding bugs found by Kimi audit

Kimi For Coding (api.kimi.com, kimi-for-coding) ran a forensic audit on
distillation v1.0.0 with full file content. 7/7 flags verified real on
grep. Substrate now matches what v1.0.0 claimed: deterministic, no
schema bypasses, Rust tests compile.

Fixes:
- mode.rs:1035,1042  matrix_corpus Some/None -> vec![..]/vec![]; cargo
                     check --tests now compiles (was silently broken;
                     only bun tests were running)
- scorer.ts:30       SCORER_VERSION env override removed - identical
                     input now produces identical version stamp, not
                     env-dependent drift
- transforms.ts:181  auto_apply wall-clock fallback (new Date()) ->
                     deterministic recorded_at fallback
- replay.ts:378      recorded_run_id Date.now() -> sha256(recorded_at);
                     replay rows now reproducible given recorded_at
- receipts.ts:454,495  input_hash_match hardcoded true was misleading
                       telemetry; bumped DRIFT_REPORT_SCHEMA_VERSION 1->2,
                       field is now boolean|null with honest null when
                       not computed at this layer
- score_runs.ts:89-100,159  dedup keyed only on sig_hash made
                            scorer-version bumps invisible. Composite
                            sig_hash:scorer_version forces re-scoring
- export_sft.ts:126  (ev as any).contractor bypass emitted "<contractor>"
                     placeholder for every contract_analyses SFT row.
                     Added typed EvidenceRecord.metadata bucket;
                     transforms.ts populates metadata.contractor;
                     exporter reads typed value

Verification (all green):
  cargo check -p gateway --tests   compiles
  bun test tests/distillation/     145 pass / 0 fail
  bun acceptance                   22/22 invariants
  bun audit-full                   16/16 required checks

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

auditor/schemas/distillation/evidence_record.ts

@@ -121,6 +121,14 @@ export interface EvidenceRecord {
   // and have no text payload. Present for distilled_*, contract_analyses,
   // mode_experiments, scrum_reviews etc.
   text?: string;
+
+  // ── Domain-specific metadata bucket ──
+  // Source-specific fields that don't earn a top-level slot. e.g.
+  // contract_analyses rows carry `contractor` here; mode_experiments
+  // could carry `corpus_set`. Typed scalar values only — keep this
+  // small or it becomes a junk drawer. Added 2026-04-27 (Kimi audit
+  // flagged `(ev as any).contractor` schema bypass at export_sft.ts:126).
+  metadata?: Record<string, string | number | boolean>;
 }
 
 export function validateEvidenceRecord(input: unknown): ValidationResult<EvidenceRecord> {