Mac M4 K2.A.1 partial evidence: KL OFF ctx70 JSON + crash log on KL ON

Mac mini K2.A.1 run on K1.H schema (Gemma 3-1B-it MPS bf16):
  ctx70 KL OFF: completed, JSON written
  ctx70 KL ON:  crashed at index_copy_ dtype mismatch (BFloat16 vs Float),
                no JSON; log captures the crash trace
  ctx280:       not attempted (script aborted after ctx70 KL ON failure)

The crash is the same _round_trip_resident_through_compressor bug
that vast K2.A.1 (17a7791) already documented and that the K2.A.1
branch has fixed at commit 66b4fbe. PR #87 cherry-picks the fix
to main. After PR #87 merges, this Mac M4 run should be repeated
on the full ladder.

The KL OFF JSON serves as the Mac M4 K2.A.1 baseline at ctx70 —
expected to match the K1.H Mac evidence (4fb947f) at ctx70 in
recall (v04 == oracle == 1.000) since K2.A.1 with KL OFF
collapses to K1.D with IdentityCompressor (no behaviour change
vs K1.D bit-for-bit, per the regression test
test_default_factory_matches_k1_baseline_bit_for_bit in #83).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 2 people