gqa(adr/ddd): iteration-2 corrections — GQA reuse via M-fold, transpose caveat

Verified against sim_engine data path (memory_store, data_executor):

- GQA reuse does NOT need a broadcast op. The baseline's h_q==h_kv limit
  is its head-packing reshape hack, not a missing primitive. Correct fix:
  per-KV-head loop with G folded into matmul M dim (byte-conserving
  reshape) — runs today, timing correct (m=G*T_q), data mode runs.
- ADR-0061 broadcast demoted from 'the blocker' to optional convenience.
- Surfaced tl.trans = reshape-not-transpose (memory_store reshapes;
  data_executor np.matmul on reshaped operands) -> numeric parity is
  bounded; verification is structural/timing/determinism-first (matches
  SPEC perf-model purpose). Optional tl.transpose deferred.
- Reordered DDD phase plan (P1 GQA needs no new feature; P3 scratch_scope
  is the key scale feature); added open decisions 10.10 (transpose) and
  10.11 (GQA-via-M-fold finding).

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

docs/adr-proposed/ADR-0061-prog-tl-broadcast.md

@@ -4,11 +4,21 @@
 
 Proposed
 
-> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention). This ADR
-> removes the single hard blocker that keeps the existing attention
-> kernels at `h_q == h_kv == 1`: there is no way to expand a KV head to
-> the `G` query heads that share it without violating the MemoryStore
-> byte-conservation check.
+> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention).
+>
+> **⚠ Re-evaluated during design iteration — this is now OPTIONAL, not a
+> GQA blocker.** The original framing (below) held that GQA `h_q > h_kv`
+> is blocked by the MemoryStore byte-conservation check on a broadcast
+> view. That is true *only of the baseline's head-packing hack*
+> (`_view(K, (h_q·d, S_kv))`). The correct GQA design (ADR-0060 §5.2)
+> processes **one KV head at a time** and folds the `G` group rows into
+> the matmul **M** dimension — using only byte-conserving reshapes — so it
+> needs **no broadcast op at all**, and `np.matmul` already broadcasts a
+> shared operand in Phase-2 data mode. This ADR therefore drops to a
+> *convenience* primitive: it cleans up additive-mask construction across
+> the `G·T_q` rows and serves general kernels. **Lowest priority of the
+> three supporting ADRs.** The rest of this document is retained as the
+> design for that convenience op.
 
 ## Context