rMLX 0.2.7

Constrained-decode hot-path and Gemma4-unified vision tuning. The json_schema
and json_object per-token allow-mask probes no longer deep-clone their grammar
across the ~152K-token vocab on every decode step, and a whitespace stall in
schema-constrained decode is fixed. Gemma4-unified gains a per-request image-token
budget. No breaking changes.

Added

• Per-request + CLI image-token budget for Gemma4-unified vision. A
  image_max_tokens request field (and matching CLI flag) caps soft image
  tokens per request; default 280, ceiling 1120. Lets callers trade vision
  fidelity for prefill cost on the unified any-to-any path. (#181, closes #180)

Fixed

• Schema-constrained decode whitespace loop. Under response_format: json_schema, enum / scalar leaves accepted insignificant whitespace in
  states where it must be rejected (inside a literal, inside a string, at the
  root scalar start), letting temp=0 decode loop on \n. The allow-mask now
  matches whitespace per-leaf-state and rejects raw control chars (0x00..=0x1f)
  inside strings. (#183)

Performance

• json_schema constrained decode no longer deep-clones the schema per
  vocab token. The allow-mask probe reset a scratch SchemaGrammar ~152K
  times per decode step, deep-copying the immutable parsed schema each reset.
  The schema is now held behind Arc (Object.props, Union, Array.items),
  so entering a container/property/union branch is a refcount bump and the
  per-token reset reuses buffers in place. Per-step cost on the production path
  drops ~8–25× (was 20–40× heavier than the json_object engine; now
  comparable). Tool / function-calling agents pay this directly. (#184,
  closes #182)
• json_object constrained decode allow-mask reset is scratch-reused. The
  JsonGrammar reset became a state copy + Vec clear/extend (the stack frame
  is Copy) instead of a fresh clone per vocab token — ~2× on the per-step
  probe. The two engines now share one fill_allow_mask kernel over a
  ProbeGrammar trait. (#183)