policy(adr): ADR-0026 Revision 6 — document DPPolicy.cube_start

Amends ADR-0026 to reflect the cube_start field added in e2fe331.
The production code shipped without an ADR update; this fills that
gap. Documentation-only change (no production code, no test code).

Changes (mirrored in both EN and KO):
- Status: Revision 5 → Revision 6
- D1: add ``cube_start: int = 0`` to the canonical DPPolicy dataclass
- D3: ``cube = policy.cube_start + cube_id`` in resolve_dp_policy
- D8: new section explaining purpose (disjoint cube sub-meshes for
      GQA Llama-70B 8-KV-group headline), semantics, default-0 backward
      compatibility, intra-device constraint, design rationale
      (scalar vs 2D origin vs cube_ids list), and the kernel-side
      cube_start subtraction needed to compensate for ADR-0022's
      physical-cube-id ``program_id(axis=1)`` semantics.

tools/verify_adr_lang_pairs.py passes (EN/KO Status keyword and
title in sync).

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

docs/adr/ADR-0026-par-dppolicy-intra-device.md

@@ -2,7 +2,7 @@
 
 ## Status
 
-Accepted (Revision 5 — Phase 2 landed 2026-04-14, 523 passed + 1 strict xfail)
+Accepted (Revision 6 — cube_start added 2026-06-04; Revision 5 landed 2026-04-14)
 
 ## Context
 
@@ -31,9 +31,11 @@ class DPPolicy:
     pe: Literal["replicate", "column_wise", "row_wise"] = "replicate"
     num_pes: int | None = None
     num_cubes: int | None = None
+    cube_start: int = 0   # Revision 6 — first cube in SIP; see D8
 ```
 
 Removed fields: `sip`, `num_sips`.
+Added fields (Revision 6): `cube_start` — see D8.
 
 ### D2. `ShardSpec` — structural (sip, cube, pe) coordinates, `pe_index` fully removed
 
@@ -119,9 +121,9 @@ def resolve_dp_policy(
 
         for ls in local_shards:
             all_shards.append(ShardSpec(
-                sip=target_sip,                   # from caller (current_device)
-                cube=cube_id,                     # local within SIP
-                pe=ls.local_pe,                   # local within cube (explicit name)
+                sip=target_sip,                              # from caller (current_device)
+                cube=policy.cube_start + cube_id,            # Rev 6: shifted by D8 cube_start; default 0 = local within SIP
+                pe=ls.local_pe,                              # local within cube (explicit name)
                 offset_bytes=cube_offset + ls.offset_bytes,
                 nbytes=ls.nbytes,
             ))
@@ -239,6 +241,53 @@ property: code that expected a global flat could otherwise silently
 receive a SIP-local result and index incorrectly — that possibility is
 eliminated.
 
+### D8. Add `cube_start: int = 0` for disjoint cube sub-meshes within a SIP
+
+**Revision 6 addition (2026-06-04)**.
+
+**Purpose**: address a disjoint cube sub-mesh within one SIP (e.g.
+cubes 8..15 alongside cubes 0..7). Required by the GQA Llama-70B
+8-KV-group headline target — two 2×4 KV-groups per SIP × 4 SIPs = 64
+cubes — where the second KV-group on each SIP must land on cubes 8..15
+instead of the default 0..7.
+
+**Semantics**: `resolve_dp_policy` returns `ShardSpec` with
+`cube = policy.cube_start + cube_id`, where `cube_id` iterates
+`0..num_cubes-1` within the launch. Selected cubes lie in
+`[cube_start, cube_start + num_cubes)` within the target SIP.
+
+**Default**: `cube_start = 0` preserves every existing call site
+bit-for-bit (`ShardSpec.cube ∈ [0, num_cubes)` as before). The CCL
+milestone bench's full-SIP `DPPolicy(num_cubes=16)` continues to
+produce cubes 0..15.
+
+**Constraint**: intra-device invariant preserved.
+`cube_start ∈ [0, cubes_per_sip)` and
+`cube_start + num_cubes ≤ cubes_per_sip`. SIP boundary crossing remains
+the job of `ahbm.set_device(rank)` (ADR-0024).
+
+**Why scalar** (not 2D `cube_mesh_origin` or arbitrary `cube_ids` list):
+consumer kernels (e.g. `_attention_mesh_mlo_2d`) assume row-major
+contiguous cubes; scalar `cube_start` pairs naturally with `num_cubes`
+(range = `[start, start + count)`); zero migration churn at default 0.
+More general designs can be added on top later if a non-contiguous use
+case appears.
+
+**Backward compatibility**: additive change with default value. No
+existing call site is forced to change. The "breaking change" stance
+in D7 applies only to the original sip/num_sips removal — `cube_start`
+does NOT break existing callers.
+
+**Kernel-side note**: kernbench's `tl.program_id(axis=1)` returns the
+physical cube id (ADR-0022), not a launch-local rank. Kernels that
+derive ring positions from `program_id(axis=1)` must subtract
+`cube_start` to recover launch-local rank when `cube_start > 0` —
+otherwise they compute out-of-bounds sub-mesh positions and deadlock.
+See `_attention_mesh_mlo_2d.py` for the reference pattern. A future
+revision of ADR-0022 could move `program_id(axis=1)` to launch-local
+rank semantics; that change would let kernels drop the explicit
+subtraction.
+
 ## Dependencies
 
 - **ADR-0024** (launcher): `set_device(rank)` and current-device scoping