Constitution compliance: add portable operation cancellation

Problem

The constitution requires every operation to support timeout and cancellation. OperationOptions currently exposes a timeout hint but no cancellation mechanism, and provider operations do not have a portable cancellation contract.

Scope

• Define a provider-neutral cancellation token/context and its interaction with timeout.
• Map cancellation to the Azure Cosmos, AWS DynamoDB, and Google Spanner SDK execution models.
• Define normalized cancellation errors, retryability, diagnostics, and race behavior.
• Cover all portable operations, not only update().

Acceptance criteria

• A caller can cancel every in-flight portable operation through per-request options.
• Cancellation stops or abandons native work as far as each vendor SDK permits and never returns success-shaped fallback data.
• Timeout and caller cancellation are distinguishable and consistently normalized.
• Shared conformance and all provider implementations/tests are updated.

Start by locating OperationOptions, the portable operation interfaces, shared conformance tests, and the Azure Cosmos, AWS DynamoDB, and Google Spanner provider implementations. Define the cancellation and timeout contract across those entry points, then verify every operation and provider distinguishes cancellation from timeout without returning fallback success data.