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>

internal/drift/drift.go

@@ -41,10 +41,6 @@ import (
 	"git.agentview.dev/profit/golangLAKEHOUSE/internal/distillation"
 )
 
-// mathLog is wrapped to localize the math import in case future
-// drift math swaps in a stable-log variant. Inlined by the compiler.
-func mathLog(x float64) float64 { return math.Log(x) }
-
 // ScorerDriftEntry is one mismatch — a historical (record, category)
 // pair where the current scorer disagrees with the persisted
 // verdict. Reasons captures the current scorer's explanation so
@@ -337,7 +333,7 @@ func ComputeDistributionDrift(in DistributionDriftInput) DistributionDriftReport
 		if cRatio < driftEpsilon {
 			cRatio = driftEpsilon
 		}
-		part := (cRatio - bRatio) * mathLog(cRatio/bRatio)
+		part := (cRatio - bRatio) * math.Log(cRatio/bRatio)
 		psi += part
 		report.Buckets = append(report.Buckets, DistributionDriftBucket{
 			Lower:         minV + float64(i)*width,