[Validation] Add reproducible CubeCL CPU and accelerator conformance runner

[code-tc]

Objective

Provide the reproducible native validation lane used to accept CubeCL HAL work on CPU and at least one designated accelerator, with exact environment identity and retained machine-readable evidence.

Scope

• Define the initial supported validation matrix after Introduce backend abstraction and minimal CubeCL backend #34 records the CubeCL runtime decision.
• Pin runner image/toolchain, Rust, CubeCL, OS packages and test/benchmark commands.
• Record device vendor/model, driver, runtime/backend, memory and relevant capabilities.
• Run shared host-versus-CubeCL correctness fixtures and collect structured results.
• Separate cold compilation, warm execution, transfer and peak-memory measurements.
• Retain logs, results and benchmark artifacts by commit.
• Detect unavailable, changed or incorrectly emulated hardware and fail/skip explicitly.
• Keep benchmark thresholds and supported-platform declarations human-owned.

Acceptance criteria

• A clean run can reproduce the CPU lane from documented commands.
• A designated accelerator lane proves actual device dispatch, not compilation or CPU emulation.
• Every artifact identifies commit, toolchain, CubeCL, OS, device, driver and backend/runtime.
• Numerical failures include operation, dtype, shape, expected/actual summary and tolerance contract without dumping sensitive data.
• Cold/warm/transfer/memory metrics are separate and machine-readable.
• Missing or changed hardware cannot be reported as a passing accelerator test.
• An independent reviewer can download and inspect retained artifacts.
• Runner credentials cannot access production or unrelated repositories/resources.

Non-goals

Choosing final performance thresholds, declaring broad hardware support, browser/WASM validation (owned by #39), or retiring OpenCL.

Tembo execution contract

automation:
  eligible: true
  executor_profile: tembo
  readiness: blocked
  primary_repository: TinyChain-Inc/ha-ndarray
  base_branch: main
  writable_scope:
    - ".github/workflows/**"
    - "scripts/validation/**"
    - "tests/conformance/**"
    - "benches/**"
    - "docs/validation/**"
  prerequisites:
    - "#34 runtime/version decision"
  concurrency_group: cubecl-validation-infrastructure
  required_commands:
    - "cargo fmt --all -- --check"
    - "cargo test --no-default-features --features cubecl"
    - "cargo test --all-features"
    - "run the documented designated-accelerator validation command introduced by this issue"
  evidence:
    cpu_lane: required
    actual_designated_accelerator: required
    exact_environment_identity: required
    structured_results: required
    retained_artifacts: required
  external_actions: designated-runner-only
  draft_pr_only: true
  completion_authority: human-infrastructure-and-HAL-review
  budget: {size: large, max_revisions: 3, max_wall_minutes: 180}

Program: #33