vectord: v2 envelope + handleMerge robustness — actions post_role_gate_v1 scrum

3-lineage scrum on 434f466..0d4f033 surfaced one convergent finding
(Opus + Kimi) and 3 Opus-only real bugs. All actioned in this
commit. Two false positives (Kimi rollback misreading, Opus stale-
comment claim) verified + rejected — both required manual control-
flow inspection to refute, matching the documented Kimi-truncation
behavior in feedback_cross_lineage_review.md.

Convergent fix — DecodeIndex lost nil-meta items:
- Envelope version bumped 1 → 2.
- New v2 field: IDs []string carries the canonical ID set
  explicitly, independent of meta map's nil-vs-{} sparseness.
- DecodeIndex accepts both versions: v2 reads from env.IDs; v1
  falls back to meta-key inference (with the documented
  limitation that nil-meta items are invisible — preserved for
  backward-compat with already-persisted indexes).
- Encode emits v2 going forward.
- 2 new regression tests:
  - TestEncodeDecode_NilMetaItemsSurviveRoundTrip: items added
    with nil metadata MUST survive Encode → Decode and remain
    visible to IDs(). Pre-fix would have yielded IDs() == [].
  - TestDecodeIndex_V1BackwardCompat: hand-crafted v1 envelope
    still decodes (proves the fallback path).

Opus-only fixes:
- handleMerge: non-ErrIndexNotFound errors at h.reg.Get(name) /
  h.reg.Get(req.Dest) now return 500 + log instead of falling
  through with nil src/dest pointers (which would panic on the
  next deref). Real bug — only the sentinel error was handled.
- internal/drift/drift.go: mathLog wrapper removed; math.Log
  inlined. Wrapper added no value (math was already imported).
- internal/distillation/audit_baseline.go: BuildAuditDriftTable's
  bubble sort replaced with sort.Slice. Idiomatic + shorter.

Rejected after verification:
- Kimi WARN "missing rollback on partial merge": misread the
  control flow. Code at cmd/vectord/main.go:404-414 does NOT
  delete from src when dest.Add fails (continue before reaching
  src.Delete). Only successful Adds trigger Deletes.
- Opus INFO "TimestampUnixNano comment references missing field":
  field exists at scripts/multi_coord_stress/main.go:128. Opus
  saw only the diff context, not the full file.

Deferred (no fired trigger):
- Opus WARN "no per-index lock during merge": no concurrent merge
  callers today (operators run merge as deliberate one-shot job).
  Worth a lock if/when matrixd or chatd start auto-triggering.

Disposition: reports/scrum/_evidence/2026-05-01/verdicts/post_role_gate_v1_disposition.md.

Build + vet + tests green; 2 new regression tests + all prior tests
unchanged.

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

STATE_OF_PLAY.md

@@ -269,6 +269,7 @@ a steady state. Future items will land here as production triggers fire.
 | (close-2) | **OPEN #2: distillation SFT export substrate** — `internal/distillation/sft_export.go`: `IsSftNever` predicate + `ListScoredRunFiles` (data/scored-runs/YYYY/MM/DD walk) + `LoadScoredRunsFromFile` + partial `ExportSft` that wires the firewall but leaves synthesis (instruction/input/response generation) as the next wave. Firewall pinning test fails if `SftNever` set changes without review. 5 new tests. The synthesis port remains on Rust at `scripts/distillation/export_sft.ts`. |
 | (close-2 full) | **OPEN #2 fully ported** (2026-05-01): `SynthesizeSft` + `LoadEvidenceByRunID` + `buildInstruction` ported byte-for-byte from `scripts/distillation/export_sft.ts`. All 8 source-class instruction templates (scrum_reviews / mode_experiments / auto_apply / audits / observer_reviews / contract_analyses / outcomes / default) match Rust output exactly so a/b validation between runtimes can diff JSONL byte-for-byte. `ExportSft` writes to `data/distilled/sft/sft_export.jsonl`. 5 additional tests including per-source-class template verification, extraction-rejection, empty-text-rejection, context-assembly, end-to-end fixture write. |
 | (close-2 lineage) | **Audit-baselines lineage ported** (2026-05-01): `internal/distillation/audit_baseline.go` mirrors Rust `audit_full.ts`'s LoadBaseline/AppendBaseline/buildDriftTable. `LoadLastBaseline` reads the most recent JSON line from `data/_kb/audit_baselines.jsonl`; `AppendBaseline` appends append-only with bufio. `BuildAuditDriftTable` flags drift `>20%` (configurable); zero-baseline and new-metric edge cases handled (no division-by-zero, no false-stable on zero→nonzero). `FormatAuditDriftTable` for stdout dumps. Generic on metric names so callers running both runtimes can pin Rust-compat names (`AuditBaselineRustCompat` constant lists them). 13 tests including last-line-wins, trailing-blank-tolerance, malformed-line-errors, threshold-boundary, zero-baseline-handling, sort-stability. |
+| (scrum) | 3-lineage scrum on `434f466..0d4f033` (post_role_gate_v1). Convergent finding (Opus + Kimi): `DecodeIndex` lost nil-meta items across persistence. **Fixed** by bumping envelope version 1→2 with explicit `IDs []string` field; v1 envelopes still load via meta-key fallback. Opus-only real bugs also actioned: `handleMerge` non-`ErrIndexNotFound` nil-deref, `mathLog` dead wrapper removed, bubble sort → `sort.Slice`. False positives rejected after verification (Kimi rollback misreading + Opus stale-comment claim). 2 new regression tests lock the v2 round-trip + v1 backward-compat. Disposition: `reports/scrum/_evidence/2026-05-01/verdicts/post_role_gate_v1_disposition.md`. |
 | (close-3) | **OPEN #3: distribution drift via PSI** — `internal/drift/drift.go`: `ComputeDistributionDrift` returns Population Stability Index + verdict tier (stable < 0.10, minor 0.10–0.25, major ≥ 0.25). Equal-width bucketing over combined min/max range, epsilon-clamping for empty buckets, per-bucket breakdown for drilldown. 7 new tests including identical-is-stable, hard-shift-is-major, moderate-detected-not-stable, empty-inputs-safe, all-identical-safe, bucket-counts-conserved, num-buckets-clamping. |
 | (close-4) | **OPEN #4: ops nice-to-haves** — (a) Real-time wall-clock for stress harness: per-phase elapsed time logged to stdout as it runs (`[stress] phase NAME starting (T+12.3s)` + `[stress] phase NAME done — 8.5s (T+20.8s)`); `Output.PhaseTimings` + `Output.TotalElapsedMs` written to JSON; (b) chatd fixture-mode S3 mock + (c) liberal-paraphrase calibration: not actioned — no fired trigger yet, would be speculative. Documented as deferred-until-need rather than ignored. |