docs: Layer 3 round 2 — append-at-tail grants, min bucket 8

Resolve the two remaining Layer 3 open questions:
- Access-grant ordering: append at tail. pub_x_index values in
  already-stored comments stay valid; no write amplification. Tail-
  is-recent positional leak is the accepted cost (vs re-shuffle which
  would force comments to reference signers by 32B pub_x bytes).
- Minimum slot bucket: 8. Singleton posts pad up to 8 so brand-new
  personas don't publish "no vouchees" headers.

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

sessions.md

@@ -65,6 +65,18 @@ See `CONTRIBUTING.md` for the protocol. See `AGENTS.md` for the Claude-specific
 
 **Stopping point**: commit `b8b38a6` (Layer 1) + new commit for Layer 2 both on branch; not merged. Awaiting Scott.
 
+### Update 2026-04-24 — Layer 3 round 2 (last two open questions)
+
+Two follow-up questions resolved:
+
+- **Access-grant slot ordering**: **append at tail** (not re-shuffle). I'd initially overcorrected to a "switch comments from index to pub_x bytes so shuffles are free" change; Scott reverted that and clarified the choice. Append-at-tail preserves `pub_x_index` stability across the post's lifetime — already-stored comments stay verifiable, no write amplification on grant. Accepted positional-recency leak (tail = newest grants).
+- **Minimum slot bucket**: **8**. Singleton/tiny-set posts pad up to 8 slots. Brand-new personas don't publish "I have no vouchees" headers.
+
+**Files touched**:
+- `docs/fof-spec/layer-2-mode2-fof-comments.md`: access-grant lead decision made explicit about append-at-tail and index stability.
+- `docs/fof-spec/layer-3-mode1-fof-closed.md`: minimum-8 floor added to padding lead decision; both open questions moved to Resolved.
+- `sessions.md`: this entry.
+
 ### Update 2026-04-24 — Layer 3 round 1 + cross-cutting padding rule (corrected)
 
 Scott talked to Opus and resolved Layer 3 open questions + introduced a unified padding rule that supersedes Layer 2 round 2's `rand(32..=128)`.