[Fusion][F1] Define deferred execution IR and explicit materialization boundaries

Objective

Introduce a backend-neutral, typed execution IR for deferred Array expressions without performing automatic fusion yet.

Scope

Operation nodes, inputs/dependencies, shape/dtype/layout metadata, device placement, pure-versus-materializing classification, graph validation, deterministic identity and explicit evaluation/materialization. Preserve eager observable behavior where the public API requires it.

Acceptance criteria

• IR schema/version and supported-node inventory are documented.
• Graph identity is deterministic for equivalent supported expressions.
• Cycles, invalid shapes/dtypes, unsupported operations and cross-device edges fail before dispatch.
• Aliasing, externally visible reads, transfers, reductions and mutation establish explicit materialization boundaries.
• IR and direct execution pass the same result/error/property fixtures.
• IR construction alone does not execute kernels or retain unbounded input data.

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/ir/**
    - src/array.rs
    - src/ops.rs
    - tests/ir/**
    - docs/**
  prerequisites:
    - "#42"
    - "#43"
  concurrency_group: array-execution-ir
  required_commands:
    - "cargo fmt --all -- --check"
    - "cargo test --no-default-features --features cubecl"
    - "cargo test --all-features"
  evidence:
    cpu_conformance: required
    accelerator_conformance: not-required
    actual_browser: not-required
    benchmark_report: not-required
    exact_hardware_and_driver_identity: not-required
  external_actions: none
  draft_pr_only: true
  completion_authority: human
  budget:
    size: large
    max_revisions: 3
    max_wall_minutes: 180

Start with prerequisites #42 and #43, then inspect src/ir/, src/array.rs, src/ops.rs, tests/ir/, and the existing docs. Define the documented IR schema and supported nodes, then cover validation, deterministic identity, materialization boundaries, and parity fixtures without executing during construction. Run cargo fmt --all -- --check, cargo test --no-default-features --features cubecl, and cargo test --all-features.