feat(xla): parameterize static context capacity

[inureyes]

Summary

Parameterize the OpenXLA static context capacity across graph emission, compiled artifacts, IREE runtime buffers, single-session generation, continuous batching, and server startup.

Changes

• Add MLXCEL_XLA_CONTEXT_CAPACITY with a compatibility default of 256 and validate explicit capacities against the signed IREE ABI.
• Derive prefill tensors, RoPE tables, masks, single/ragged KV caches, slot buffers, and decode bounds from one canonical capacity.
• Include capacity in compiled MLIR/VMFB artifact names and hashes, and validate returned native KV shapes before accepting them.
• Centralize text and future post-expansion multimodal admission around effective_prompt_len + max_new_tokens <= context_capacity with typed errors before scheduler mutation.
• Add 256/1024 emitter schema coverage, admission boundary and overflow coverage, a no-slot-mutation regression, operator documentation, and native Rust/C bounds checks.

Validation

• cargo fmt --all -- --check
• cargo clippy -p mlxcel-xla --lib --tests -- -D warnings
• cargo check -p mlxcel-xla --lib
• cargo check -p mlxcel --features xla-backend --lib
• IREE_DIST=/home/inureyes/Development/mlxcel/spike/iree-ffi/iree-dist cargo check -p mlxcel --features xla-iree --lib
• cargo test -p mlxcel-xla --lib context (9 passed, including 256/1024 graph schemas and admission boundaries)
• cargo test -p mlxcel-xla --lib admission
• cargo test -p mlxcel-xla --lib rejected_request_does_not_mutate_queue_or_live_slot
• cargo test -p mlxcel-xla --lib from_json_reproduces_bundled_assets_byte_for_byte
• MLXCEL_XLA_CONTEXT_CAPACITY=256 IREE_DIST=/home/inureyes/Development/mlxcel/spike/iree-ffi/iree-dist cargo run --features xla-iree --example xla_oracle_check -- --model /home/inureyes/models/smollm-135m-4bit --oracle /tmp/issue857-smollm-oracle.json --device local-task (Linux aarch64 CPU local-task, 2-token HF fp32 oracle, TOKEN-EXACT PASS)
• MLXCEL_XLA_CONTEXT_CAPACITY=1024 IREE_DIST=/home/inureyes/Development/mlxcel/spike/iree-ffi/iree-dist cargo run --features xla-iree --example xla_oracle_check -- --model /home/inureyes/models/smollm-135m-4bit --oracle /tmp/issue857-smollm-oracle.json --device local-task (TOKEN-EXACT PASS; distinct -c1024- artifacts from the -c256- pair)

The full crate-wide library suite and validate_arch.sh module-wide structural gate are deferred to the epic integration wave; the issue branch uses the focused tests above to avoid duplicating broad wave validation.

Closes #857