gqa(adr): review fixes — unify KV placement to contiguous, ring/reduce cost model, opt3 wording, rank def

Per review:
- Placement unified to *contiguous* C×P blocks throughout (S0/S2.1/S2.2/S0.5);
  round-robin demoted to the rejected alternative. Adds driver SP-enable
  threshold fallback (smaller C / C=P=1 for short/early decode).
- Ring-vs-reduce cost model in S5.5: reduce ~ G*T_q*log(C*P) (O dominant;
  m,l scalars), ring ~ 2*S (total K+V bytes a CUBE injects over C-1
  rotations; recv_async pipelines so latency ~ max-step) -> ring wins when
  T_q > 2S/(G*log(C*P)).
- opt3 'removes the bubble' -> 'hides (subject to scheduler+engine balance)'
  everywhere; table 'hidden*' with footnote.
- 'rank' defined (SP participant = a PE in a CUBE, KV shard in its HBM->TCM).
- out-proj handoff contract (S0.5.4); S11 gate-type note (absolute latency
  deferred to ADR-0064; structural + relative-to-baseline gates now).
- greenlet-as-contrast tightened to 'primitive-op (tl.dot) path' (S1, SB).
KO mirror synced; DDD gets the SP-enable threshold fallback. Docs only.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/adr-proposed/DDD-0060-gqa-fused-attention-detailed-design.md

@@ -130,6 +130,13 @@ Placement via `DPPolicy(..., cube_start=...)`:
 (ADR-0060 §2.1). `cube_start` offsets the group's CUBE sub-mesh within the
 4×4 SIP mesh.
 
+**Driver SP-enable threshold (fallback).** Contiguous sharding under-utilizes
+ranks for short/early decode (only frontier ranks hold data). So the driver
+**enables SP only past a context-length threshold** where KV-sweep time
+dominates reduction + placement imbalance; below it, fall back to a smaller
+`C` (or `C=P=1`, single rank, no reduce). The threshold is a sweep item
+(P7 / §9), not a fixed constant.
+
 `--device` enumerates **SIPs**; one SIP-device runs its 2 CUBE Groups
 (2 KV heads); the head is picked by CUBE coordinate (`head_of_group`), not an
 in-kernel loop (ADR-0060 §0).