Add source-level debounce and throttle wrappers

## Summary

Add reusable source-level debounce and throttle wrappers for subscriptions.

High-frequency sources currently need application-specific coalescing. Runtime load control bounds memory and batch work, but it is not a substitute for reducing redundant input at the source. The time model is now settled: use Tokio's executor clock and paused time for deterministic tests rather than introducing a public clock abstraction.

## Required semantics

Before exposing the API, define:

- leading and trailing behavior;
- what value is retained when several inputs arrive in one window;
- subscription identity and restart behavior;
- cancellation, scope teardown, and shutdown behavior;
- behavior when the upstream source finishes;
- mapping and composition behavior.

The wrappers should be additive stream/source transformations. They must not change kernel batching, data-lane ordering, or backpressure contracts.

## Acceptance criteria

- [ ] Public source/subscription wrappers provide debounce and throttle behavior with documented semantics.
- [ ] Existing subscription identity remains structural and stable.
- [ ] Paused-time tests cover window boundaries, bursts, upstream completion, cancellation, and restart.
- [ ] Tests cover mapping and reducer composition.
- [ ] Documentation distinguishes source-side coalescing from `batch_max_messages` and data-lane capacity.