Add GPU bitmask expansion for constrained sampling

MPSGraph subgraph that expands packed Int32 xgrammar bitmasks into
Float16 logits masks entirely on GPU. Blocked tokens get -65504
(Float16 min), ensuring they are never selected by argmax or topK.

Both MPSGraphArgmaxSampler and MPSGraphCompositeSampler gain:
- bitmaskBuffer (lazy): shared-memory MTLBuffer, allocated on first use
- constrainedExecutable (lazy): compiled graph with bitmask input,
  built on first applyBitmask: true call
- encode(..., applyBitmask:) overloads on all encode paths

Zero overhead for unconstrained generation — no bitmask buffer
or constrained graph is compiled until the first constrained call.

Existing protocol methods and behavior unchanged.

Part 2 of 4 for GPU-based constrained sampling (#114).

Author: stikves