[Fusion][F2] Implement fusion legality, CubeCL lowering, and bounded plan cache

[code-tc]

Objective

Fuse compatible IR subgraphs at runtime, lower them to CubeCL kernels, and cache compiled plans with deterministic keys and bounded storage.

Acceptance criteria

• A documented legality table covers operations, layouts, broadcasting, aliases, devices, errors and materialization boundaries.
• A representative chain of at least three compatible operations dispatches one kernel.
• Fused/unfused differential and property tests agree for results and typed failures.
• Unsupported or illegal graphs visibly fall back to unfused execution.
• Plan keys include graph, shape/dtype specialization, runtime/device capabilities, CubeCL/compiler versions and relevant options.
• Hits, misses, compile time, eviction, invalidation and fallback are observable; memory/disk growth is bounded.
• Fusion can be disabled to provide an independent oracle.

Parent: #37

Tembo execution contract

automation:
  eligible: true
  executor_profile: tembo
  readiness: blocked
  primary_repository: TinyChain-Inc/ha-ndarray
  base_branch: main
  writable_scope:
    - src/fusion/**
    - src/ir/**
    - src/cubecl/**
    - tests/fusion/**
    - benches/**
  prerequisites:
    - "#50 designated accelerator validation lane"
    - "#46"
    - "#42"
    - "#43"
  concurrency_group: array-fusion-runtime
  required_commands:
    - "cargo fmt --all -- --check"
    - "cargo test --no-default-features --features cubecl"
    - "cargo test --all-features"
  evidence:
    cpu_conformance: required
    accelerator_conformance: required-on-designated-runner
    actual_browser: not-required
    benchmark_report: required
    exact_hardware_and_driver_identity: required
  external_actions: none
  draft_pr_only: true
  completion_authority: human
  budget:
    size: large
    max_revisions: 3
    max_wall_minutes: 180