rMLX 0.2.1

Correctness + maintenance release. Closes a systemic KV-cache head-scramble
class that affected every flat quantized KV codec, hardens the SSD KV tier
and prompt cache, makes the single-MLX Metal claim self-heal after a crashed
holder, and unifies the per-architecture model code onto shared seams (decode
loop, loader, Architecture dispatch). Plus a round of dependency bumps. No
breaking changes.

Fixed

• Systemic KV head-scramble class. Every flat quantized KV codec wrote its
  buffer sequence-major but reshaped it head-major on dequant — agreeing only
  when batch × kv_heads == 1, and scrambling per-head K/V on any multi-append
  (decode after a multi-token prefill, or after an SSD hydrate) when
  kv_heads > 1 (grouped-query attention). Fixed family-wide with a canonical
  sequence-major layout (transpose on append + on dequant) and an explicit
  Array::contiguous before each custom MSL kernel, which reads its input by
  raw linear index and so cannot honor a lazy transpose. Covers QuantK
  (#103), QuantV / TurboSym-K / paged-K handoff (#108), the Iso/Rotor
  rotation codecs (#109), and PlanarQuant K/V plus its packed-K decode kernels
  (#110).
• SSD KV tier. Spill + restore now carry the bf16 K/V payload for
  KvQuant::None layers (#88); SSD-hydrated entries are excluded from the
  exact-hit fast path so a hydrate cannot be mistaken for an exact prompt-cache
  hit (#87); a Gemma 4 entry hydrated with an empty SWA layer degrades to a
  full re-prefill instead of decoding from a hole (#90).
• Prompt cache unified across architectures. A single model-agnostic
  consume engine replaces the per-arch hydrate/reuse glue and is retrofitted
  onto five architectures, so the SSD-hydrate / prefix-reuse correctness fixes
  above hold identically on every model (#98).
• GPU default stream on every inference entry. The image, speculative,
  audio, and embeddings blocking-thread entries now establish the thread-local
  GPU stream the text path already had, fixing intermittent
  no Stream(gpu, N) failures off the text path (#104). The adaptive
  prefill-chunk fallback resolves the loaded architecture instead of assuming
  Gemma 4 (#68).
• Metal claim self-heals. A stale claim left by a crashed holder is
  auto-reclaimed once the holder PID is proven dead (re-probed under the file
  lock); SIGTERM/SIGINT now shut the server down gracefully and release the
  claim (#112).
• Array::to_bytes evaluates before reading the data pointer, closing a
  lazy-eval race in the only reader of the raw MLX array buffer (#101).
• MetalKernel::new frees its input vector when output-name conversion fails
  (#60); the Planar3 V codec uses one packing path on CPU and GPU (#102) and
  warms its MSL kernels at precompile (#59); the resident-bytes estimator
  models Iso/Rotor sidebands exactly (#58); chunked_prefill exits prefill on
  every cache after a failure (#57); f16 negative subnormals no longer decode
  to -0.0 (#56); the tensor-view loader distinguishes not-found from I/O /
  parse failures (#4b5ea54 → see history).
• Gemma 4 loading: unquantized bf16 and affine-int4 (QAT) snapshots load;
  affine biases pass through the MoE expert gather_qmm; the perplexity scorer
  prepends BOS to every sliding window.

Changed

• Shared decode loop. Qwen 3, Qwen 3.5-MoE, Gemma 4, and Gemma 3 now run on
  one decode loop (per-arch copies removed); ProbeStep / SmokeVerdict live
  in the shared loop.
• Shared loader seam. All architecture loaders (Gemma 4 / 3, Qwen 3 /
  3.5-MoE / 3-VL-MoE, Laguna) adopt load_util::Weights — an index-first,
  header-truth tensor fetch; AWQ byte-math moved to rmlx-quant; a single
  read_raw_config helper replaces six per-loader clones.
• Architecture dispatch. Auto-KV default, KV-byte reporting, and
  prompt-cache stats now dispatch through the Architecture trait rather than
  arch-specific branches.
• Shared fused-QK setup scaffold (q8 / turbo-K3 / turbo-K4 / iso dispatchers
  ported onto it); arch modules construct arrays via
  Array::from_{i32,f32}_slice per docs/FFI.md; refuses_qwen_moe renamed to
  k_below_8bit (it is a codec property, not an arch rule).

Dependencies

• tokenizers 0.20 → 0.23 (encode/decode add-special / skip-special semantics
  preserved; verified on Gemma 4, Qwen 3.6, and Bonsai tokenizers) (#97).
• toml 0.8 → 1.1 (#94), tikv-jemallocator 0.6 → 0.7 (#96),
  criterion 0.5 → 0.8 (dev / benches) (#95), uuid 1.23.2 → 1.23.3 and
  time 0.3.47 → 0.3.49 (#93).

Tested

• Full KV-codec regression re-sweep after the head-scramble fixes: every codec
  class (QuantK/V, Iso/Rotor, Planar including its live fused-QK kernel) is
  within ±5 % of its recorded best decode cell on Bonsai, Gemma 4-e4b, and
  Qwen 3.6 — no decode regression. GPU round-trip tests assert each layout flip
  reconstructs true head-major K/V at quant noise (with pre-fix scramble
  controls).
• Tokenizer correctness re-proven on three tokenizer families (SentencePiece +
  BPE) at temp 0.