[Streaming][S1] Define ordered Array-block stream contract and bounded backpressure

Objective

Define and implement the backend-neutral asynchronous contract for a fallible stream of Array blocks before adding fused CubeCL execution.

Contract

Specify stream dimensions, compatible dtype/rank/shape variation, input/output ordering, partial progress, upstream/downstream errors, cancellation, consumer drop, maximum in-flight blocks and host-memory accounting.

Acceptance criteria

• Public API and compatibility/version policy are documented.
• Slow-consumer tests prove bounded buffering and effective backpressure.
• Ordering and terminal/error semantics are deterministic.
• Cancellation and downstream drop stop upstream work and release buffers.
• An incompatible block returns a typed error identifying its sequence and violated metadata.
• Sequential host execution is the independent result oracle.

Parent: #38

Tembo execution contract

automation:
  eligible: true
  executor_profile: tembo
  readiness: blocked
  primary_repository: TinyChain-Inc/ha-ndarray
  base_branch: main
  writable_scope:
    - src/stream/**
    - src/array.rs
    - src/buffer.rs
    - tests/stream/**
    - docs/**
  prerequisites:
    - "#41"
  concurrency_group: array-stream-contract
  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 by checking prerequisite #41, then inspect src/stream/, src/array.rs, src/buffer.rs, tests/stream/, and docs/** to locate the existing stream and block metadata entry points. Run the required cargo fmt and test commands to establish the baseline. Done means the documented contract, bounded-buffer and cancellation tests, deterministic error behavior, typed incompatibility errors, and sequential host oracle satisfy the acceptance criteria.