* chore: bump version to 0.6.51

User-facing changes since v0.6.50:

* fix(prefix-cache): slice 3D KV state along the right axis (#392) —
  inference correctness fix for prefix cache restore on KV tensors
  shaped (B, H, S) (was slicing the wrong axis, surfacing as
  silent first-token corruption on cache-hit prompts).
* perf(scheduler): drop per-decode list() copy of output_token_ids
  (#391) — small per-token allocation removed from the decode hot
  path; benefit scales with output length on multi-stream batches.
* feat(json-output): strip spurious backslashes before non-ASCII
  chars (#394) — JSON-emitting models occasionally emit `\\é` etc.
  during constrained generation; now stripped before delivery so
  `json.loads` doesn't choke on user output.

Dev/docs (no user-visible runtime change):
* docs(contributing): codify test precision policy correctness=8bit /
  perf=4bit (#396)
* chore(pr_validate): swap Qwen3.6-27B-4bit → 8bit in stress matrix
  with smoke-tier 4-bit fallback for ≤32 GB hosts (#395)

Pre-merge artifact SHA-256 (audit anchor; will not match post-publish
SHAs because publish.yml rebuilds on Linux runners — see Release SOP §8):
  rapid_mlx-0.6.51-py3-none-any.whl
    5affa5c527bf543b72ddab94e96f2c6308225e6f5facd229d8f21f0b9ede8ce7
  rapid_mlx-0.6.51.tar.gz
    d248b8a7754cbe4d94b4708fa0fb88caa623f7fc71f13f156ece0a3b36224755

Release SOP gates:
* §3 install size: 448 MB (vs 445 MB v0.6.48 baseline, +0.67% — well
  under 1.05× soft-warn threshold).
* §7 supply chain: pip-audit clean on critical deps via OSV; recent
  uploads (HF hub 1.15.0 today, transformers 5.8.1 3 days) verified
  legit upstream (substantive changelogs, known maintainers / HF bot);
  no install.sh / workflows / pyproject.toml diffs since v0.6.50;
  OIDC scope minimal (id-token: write only on publish, contents: write
  only on auto-release); 3 third-party actions still moving-tag pinned
  (peter-evans, peaceiris, codecov) — pre-existing, not a release
  blocker but tracked for follow-up.
* §5 perf: make full in flight (qwen3.5-35b done; qwen3.6-35b mid-run).
  Will report results in PR before merge.
* §4 user-onboarding personas + §6 agent smoke: pending; will run
  serially after §5 (per CLAUDE.md "never in parallel" guidance for
  model-server workloads).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* fix(api): detect text-only forks of multimodal architectures (#393)

Issue #393 reports that ``rapid-mlx serve /path/to/Qwen3.6-35B-A3B-MLX-8bit``
crashes on startup because the model is routed to the MLLM batched
engine even though the user's checkpoint is text-only. Reporter (Tylast)
correctly identified the chain: the checkpoint's config.json declares
``vision_config`` (because the base ``Qwen3_5MoeForConditionalGeneration``
architecture is multimodal-capable), so ``is_mllm_model`` returns True,
so the MLLM loader takes over, then hits the hybrid-backbone /
ArraysCache incompatibility documented in the closed-as-spam #385.

The text-only A3B fork ships zero vision tensors in its safetensors,
even though its config.json carries the full vision_config block. Our
detection trusted the config and ignored the actual weight presence.

Fix: when config indicates VLM AND the path is a local directory AND
that directory ships ``model.safetensors.index.json``, scan the index
for tensor-name prefixes that indicate real vision/audio weights
(``vision_tower``, ``visual.``, ``audio_tower``, ``mm_projector``, …).
If none are present, override to text-only routing. The check fires
only in the True → False direction; the False direction is preserved
as-is to keep existing text-routed models stable.

Conservative on edge cases:
* Single-file safetensors (no sharded index) → return None from the
  probe and trust config. Wrong-True here means the text path errors
  clearly at first image request, whereas wrong-False would silently
  corrupt every text request on a real VLM. The bad-direction cost
  is much smaller.
* Unreadable / oversized / malformed index → same. Fall back to config.
* HF repo IDs (not local dirs) → unchanged; we'd need a network call
  to inspect remote tensors.

Tests:
* New ``TestIsMllmModelWeightsPresenceOverride`` class — 6 cases:
  - vision_config + no vision tensors → False (the #393 fix path)
  - vision_config + vision tensors → True (genuine VLM still works)
  - audio_config + audio tensors → True (audio branch covered)
  - missing index → fall back to config
  - malformed index → fall back to config
  - text-only config → never even probe weights

Total tests: 102 pass (was 96). ruff clean.

This change is bundled into the v0.6.51 bump because the user-facing
fix is small + isolated and waiting for v0.6.52 would mean Tylast
keeps hitting the crash for another release cycle.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* docs(install): add brew 5.x homebrew/core pre-flight hint

Brew 5.x's install sandbox cannot auto-tap homebrew/core mid-install
when a third-party formula depends on core packages ([email protected], rust).
Users on fresh brew installs (API-only, no homebrew/core tap cloned)
see "Operation not permitted" on /opt/homebrew/Library/Taps/homebrew/.

Pre-tapping with `brew tap homebrew/core --force` (one-time, ~1.3 GB)
lets the install complete. Brew 4.x and earlier never needed this.

---------

Co-authored-by: Your Name <[email protected]>
Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>