feat(xla): bind operator numeric contracts

[inureyes]

Summary

• define a stable per-operator numeric descriptor covering checkpoint encoding, input/result materialization, accumulator dtype, rounding boundaries, association policy, exact affine dequantization order, and backend identity
• add auxiliary artifact schema v3 and bind the operator descriptor fingerprint to write, load, mismatch rejection, and downgrade protection
• preserve the exact v1/v2 artifact digests and make an existing diagnostics-only Gemma3n SDPA ABI test runnable on CPU-IREE

Safety boundary

• no production family switches from new_legacy_unqualified
• no CLI/server capability predicate changes
• no model or operator oracle pass is claimed by this descriptor-only slice
• shared emitter helpers and deterministic micro-oracles remain follow-up work in feat(xla): define operator-level MLX/IREE numeric contracts #932

Validation

• cargo check -p mlxcel-xla
• cargo test -p mlxcel-xla --features iree operator_numeric_contract -- --nocapture (4 passed)
• cargo test -p mlxcel-xla --features iree aux_manifest::tests -- --nocapture (14 passed)
• cargo test -p mlxcel-xla --features diagnostics sdpa_production_abi_passes_dynamic_position_and_exact_scale_bits -- --nocapture (1 passed)
• cargo-clippy clippy -p mlxcel-xla --features diagnostics --tests --no-deps (passed; existing warnings only)
• cargo-fmt fmt --all -- --check

Refs #932