feat: add Youtu-VL XLA vision and MLA inference (#872)

[inureyes]

Summary

• Add an explicit Youtu-VL XLA text configuration that maps native dense DeepSeek-style MLA factors, compressed KV cache rows, and traditional interleaved RoPE across token/embedding prefill, single decode, and ragged continuous batching.
• Add static 16/64/256-patch IREE vision buckets with checked grid planning, 2D RoPE, per-image/window attention masks, full-attention block selection, output-order restoration, and the checkpoint built-in merger without an extra projector.
• Add canonical filtered loading and host preprocessing that enforces processor patch caps before allocation/upload, replaces configured image placeholders, preserves video-placeholder fallback only when image placeholders are absent, and rejects silent MLX fallback.
• Bind processor identity, spatial/window/merger configuration, checkpoint weight schema, placeholder IDs, MLA decomposition, RoPE flags, and static limits into runtime compatibility boundaries.

Validation

• cargo check --features xla-iree
• cargo test -p mlxcel-xla --lib youtu_vl (focused XLA plan/emitter tests passed)
• cargo test --features xla-iree --lib youtu_vl (21 passed)
• cargo test --features xla-iree --lib youtu_vl_placeholder_selection (1 passed)
• cargo test -p mlxcel-xla --lib youtu_vl_selects_dense_mla_and_interleaved_rope (1 passed)
• Pinned Youtu-VL MLA token prefill, single decode, and ragged decode graphs compiled successfully with the pinned IREE compiler.
• One pinned-checkpoint local-task IREE vision execution completed successfully with finite [4, 2560] merged output from the 16-patch bucket.
• cargo clippy --features xla-iree --lib --no-deps
• Central review follow-up e3383695 rejects unsupported attention_bias=true before MLA emission; the focused parser regression passed.
• cargo test --lib reverse_indices_restore_original_group_order (1 passed with a non-self-inverse window permutation)
• cargo fmt --all -- --check
• git diff --check

Draft limitation

• The checkpoint smoke test validates one real 16-patch local-task IREE execution, but a full deterministic token-level comparison against an independent reference oracle is not yet available; keep this PR in draft until that boundary is established or explicitly waived.

Test-suite note

• The complete mlxcel-xla library suite has one pre-existing unrelated DeepStack C ABI assertion failure; 231 tests passed, 11 were ignored, and all focused Youtu-VL tests passed.

Closes #872