Move paced mutation hooks to a shared adapter conformance contract #2178

Problem

Only @tanstack/react-db currently exposes a usePacedMutations framework hook. Its callback freshness rules therefore live in the React adapter, and PR #2170 needs a React-specific oracle for rendered-versus-committed callbacks.

If equivalent hooks are added to Vue, Svelte, Solid, or Angular, duplicating this policy in each package would make callback ownership and pacing behavior drift.

Proposed direction

• Define a shared paced-mutation adapter contract for callback ownership, stable mutation-function identity, strategy timing, and transaction settlement.
• Keep framework lifecycle semantics in receiving drivers. React must distinguish rendered from committed callbacks; other frameworks should define their corresponding activation/publication cut.
• Move the independent active-callback-owner model into the shared conformance suite when there is another framework consumer.
• Keep core createPacedMutations responsible for pacing, transaction admission, ordering, and settlement rather than framework lifecycle.

Current boundary

Until another framework exposes an equivalent hook, PR #2170 and its executable oracle remain in packages/react-db. The core paced-mutations oracle does not claim framework render scheduling.

Acceptance criteria

• At least two framework adapters consume one shared contract.
• Each driver names its lifecycle publication cut and tests abandoned/uncommitted work where applicable.
• Callback ownership, mutate identity, and immediate versus delayed persistence have shared expected outcomes.
• The oracle coverage map names the shared owner and each receiving driver without lending one framework evidence to another.