refactor(chain)!: split canonicalization into two tasks with generic `Canonical<A...

refactor(chain)!: split canonicalization into two tasks with generic `Canonical<A, P>`

Separate concerns by splitting `CanonicalizationTask` into two phases:

1. `CanonicalTask` determines which transactions are canonical and why
   (`CanonicalReason`), outputting `CanonicalTxs<A>`.
2. `CanonicalViewTask` resolves reasons into `ChainPosition`s (confirmed
   vs unconfirmed), outputting `CanonicalView<A>`.

Make `Canonical<A, P>`, `CanonicalTx<P>`, and `FullTxOut<P>` generic over
the position type so the same structs serve both phases. Add
`LocalChain::canonical_view()` convenience method for the common two-step
pipeline.

Renames:

- `CanonicalizationTask` -> `CanonicalTask`
- `CanonicalizationParams` -> `CanonicalParams`
- `canonicalization_task()` -> `canonical_task()`
- `FullTxOut::chain_position` -> `FullTxOut::pos`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>