Merge bitcoindevkit/bdk#2246: feat(chain)!: taint-aware `CanonicalView::balance`

e2dad373b0e2023a58c67afb073406eeaf028374 bench(chain): benchmark classify_outpoints across canonical topologies (Dmenec)
01b466018c98af9aa3b7335b31c1ea23de0e3f42 test(chain): cover taint propagation and eligibility classification (Dmenec)
3f7c1bde852826cd3df0a12dd0ae8023a6f0bb07 feat(chain)!: derive balance trust from output ancestry (志宇)
98a7d5b88e975d529a4c8e6446ef8c3a4314281e feat(chain): classify outpoints by chain-level spend eligibility (Dmenec)

Pull request description:

  Solves #2267

  ### Description

  Takes over #2235 (thanks @evanlinjin for the go-ahead). It reworks `CanonicalView::balance` to derive trust from an output's unconfirmed ancestry, and adds `classify_outpoints`, a per-output spend-eligibility classifier that `balance` becomes a thin fold over.

  The old `balance` decided trust using a per-output `trust_predicate`, which cannot express transitive trust. As a result, owned outputs whose unconfirmed ancestry included foreign coins were counted as trusted. That is the root cause of the wallet trust-classification bugs (bitcoindevkit/bdk_wallet#16, bitcoindevkit/bdk_wallet#273).

  The API now takes two separate predicates, one per concern:

  - `does_taint(&tx)` - should this transaction be considered tainted? (e.g., because it spends a foreign unconfirmed output)
  - `is_settled(&pos)` - do we consider this chain position settled / final? (generalizes `min_confirmations`)

  For each unspent output, `classify_outpoints` reports its chain-level spend eligibility:

  - `Settled` if considered settled according to `is_settled`
  - `Immature` for a coinbase output that has not yet matured
  - `Unsettled(Trust)` otherwise, where `Trust` is:
    - `Trusted` if the whole unconfirmed ancestry only spends owned coins
    - `Untrusted` if the output itself, or any unconfirmed ancestor, is tainted
    - `Unknown` if part of the ancestry is missing from the view, so trust can't be determined

  `balance` then sums each output's value into the bucket corresponding to its `Eligibility`.

  ### Notes to the reviewers

  - Trust is resolved through a self-contained ancestry walk. The traversal is memoized: each visited transaction is cached so that already-classified transactions (and their ancestors) do not need to be walked again.
  - Kept `Balance::confirmed` and did not rename it to `settled`. That rename is out of scope here. The folding logic is slated to move into `bdk_wallet`, and the current `balance` function in chain will eventually be deprecated.
  - `balance` also drops the `O` generic and now takes plain `OutPoint`s, since the taint predicate operates on transactions rather than per-outpoint associated data.
  - `does_taint` is evaluated at most once per transaction.
  - More sophisticated classification rules (e.g., for coin control or locked funds) can be built on top of `classify_outpoints`. Left for a follow-up.

  ### Changelog notice

  - **Breaking**: `CanonicalView::balance` now takes `does_taint: impl FnMut(&CanonicalTx) -> bool` and `is_settled: impl Fn(&ChainPosition) -> bool` instead of a per-output trust predicate and `min_confirmations`, and plain `OutPoint`s instead of `(identifier, outpoint)` pairs (this drops the `O` generic). Trust is now derived from an output's unconfirmed ancestry.
  - **Breaking**: added `Balance::unknown_pending`, where outputs whose trust can't be determined are counted instead of being lumped into `untrusted_pending`.
  - Added `CanonicalView::classify_outpoints` and the `Eligibility` enum.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `just p` before pushing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

  #### Bugfixes:

  * [x] This pull request breaks the existing API

ACKs for top commit:
  evanlinjin:
    ACK e2dad373b0e2023a58c67afb073406eeaf028374
  nymius:
    ACK e2dad373b0e2023a58c67afb073406eeaf028374

Tree-SHA512: 30db16f27b3068f7ed2e0ac43b199438437622a66ff1a1a8d9fd854d22895384979a3e2305139ca7955137087bde832c64d31807069f60f08293c5e194234693