Refactor ccl_allreduce bench: rank=SIP only, remove rank=PE legacy path

The unified ccl_allreduce bench previously carried two execution models
in one worker with ``if world_size == n_sips:`` branching:
  - TP mode (rank = SIP, ADR-0024/0027): proper ProcessGroup semantics.
  - Legacy rank = PE mode: single-driver worker allocating one big tensor
    distributed across all PEs via _derive_dp, with kernel-level SPMD via
    program_id.

The second model is unnecessary — intra-SIP PE-level collectives are
expressed inside the kernel (tl.send/tl.recv with program_id, IPCQ) and
do not need a host-side ProcessGroup. Removing it lets the bench be a
clean reference implementation of the TP launcher.

benches/ccl_allreduce.py:
- Config resolved once in run() via _resolve_cfg -> _BenchCfg dataclass.
- rank != n_sips now raises RuntimeError explicitly.
- _worker / _allocate_rank_tile / _init_with_rank_value / _report each
  have one concern; duplicated init + verification paths collapsed.
- _derive_dp and the second verify+print block deleted.
- 166 lines -> 91 lines.

ccl.yaml:
- mesh_allreduce_4 (world_size: 4) and tree_allreduce_7 (world_size: 7)
  algorithm entries removed (rank = PE only).
- Algorithm kernel files (kernbench.ccl.algorithms.mesh_allreduce,
  tree_allreduce) kept as-is for direct-dispatch future use.

tests/test_ccl_allreduce_matrix.py:
- Matrix shrinks from 7 cases to 3: ring × {tcm, hbm, sram} at ws =
  topology SIP count (= 2). mesh_2x2, tree_binary_7, ring_multi_cube,
  and the three ring_*_8 cases removed.

tests/test_ccl_performance.py:
- _run_8rank renamed to _run_ring; world_size: 8 override dropped; now
  exercises rank = SIP ring all-reduce.

tests/test_mp_spawn.py, tests/test_ccl_ddp_launcher.py:
- Monkeypatch target updated from bench.worker to bench._worker
  (signature now takes BenchCfg instead of (rank, world_size)).

555 passed, 1 intentional skip. Tests that directly call
install_ipcq(world_size_override=N) for kernel-level sanity
(test_ccl_hello_world_guide, test_recv_copy_to_dst, test_tl_recv_async,
test_ccl_deadlock_detection) are unchanged — they never went through
the bench and still exercise the kernel-only path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_mp_spawn.py

@@ -155,10 +155,10 @@ def test_ccl_allreduce_hand_rolled_loop_replaced_by_mp_spawn(
 
     calls: list[tuple[int, int]] = []
 
-    def _fake_worker(rank, world_size, torch):
-        calls.append((rank, world_size))
+    def _fake_worker(rank, cfg, torch):
+        calls.append((rank, cfg.world_size))
 
-    monkeypatch.setattr(bench, "worker", _fake_worker)
+    monkeypatch.setattr(bench, "_worker", _fake_worker)
 
     with _make_ctx(topology) as ctx:
         bench.run(ctx)