0.10: Implement native async and cancellation

[bazer]

Objective

Add a coherent native asynchronous and cancelable execution surface across DataLinq queries, explicit relation loads, mutations, transactions, and provider metadata operations while preserving the existing synchronous contract.

Required scope

• Complete the W0 I/O and public-surface audit tracked by 0.10: Produce release evidence and frozen-candidate closeout #106 before freezing public APIs.
• Define internal async provider/access/source contracts and deterministic cancellation/failure semantics.
• Implement provider async paths for SQLite, MySQL, and MariaDB; use native asynchronous I/O only where the underlying provider genuinely supports it and document SQLite driver limitations explicitly.
• Add audited public async operations for supported query terminals, sequence/scalar materialization, explicit relation loads, mutations, transactions, and schema metadata reads.
• Accept and propagate CancellationToken at meaningful public I/O boundaries.
• Distinguish caller cancellation, timeout, provider failure, rollback failure, and uncertain commit outcomes.
• Preserve sync/async result, conversion, cache, invalidation, logging, metrics, invocation-snapshot, disposal, and transaction-terminal parity.
• Keep synchronous APIs as direct synchronous implementations.

Acceptance criteria

• Representative sync and async operations return the same values and observable telemetry shape.
• Cancellation before dispatch, during provider execution/materialization, and during multi-step orchestration has deterministic tested behavior.
• Reader, command, transaction, and disposal lifetimes are covered for SQLite and the supported MySQL/MariaDB targets.
• No provider call with a genuine native async equivalent is routed through Task.Run or sync-over-async.
• Mutable query invocation values are captured at the existing snapshot boundary before any await.
• Public additions have XML/API documentation, consistent token placement, ApiCompat review, and consumer-shaped coverage.
• Focused A10 evidence is recorded in 0.10: Produce release evidence and frozen-candidate closeout #106 before this issue closes.

Dependency and order

• Blocked on the RE10-0/W0 baseline in 0.10: Produce release evidence and frozen-candidate closeout #106.
• Internal contracts precede provider work; provider feasibility precedes the public async surface.
• The stable public async/cancellation contract blocks final H10 hosting/unit-of-work design and feeds V10 and T10.

Explicit non-goals

• Awaitable entities, automatic lazy loading, or synchronous property access that performs hidden I/O.
• Async APIs that merely wrap synchronous provider calls.
• A general backend plugin API.
• Unrelated query-language or mutation expansion, including Support atomic conditional updates with affected-row counts #65.

Planning sources

• 0.10 A10 roadmap contract
• Implementation order W1-W3
• Durable async design plan