Merge bitcoindevkit/bdk#2074: Fix `ChainPosition` ordering

1cc7d606ac55fafff1ccf5bcc7ddabb2c723805e fix(chain): correct unconfirmed `ChainPosition` `last_seen` tiebreaker (志宇)
184db01a38a8970d1628258e22a24941fcbd3a3f fix(chain): correct ChainPosition ordering for wallet transaction display (志宇)

Pull request description:

  ## Description

  This PR fixes incorrect ordering of `ChainPosition` that affected how wallet transactions are displayed. Previously, unconfirmed transactions that were never seen in the mempool would incorrectly appear before transactions with mempool timestamps due to the derived `Ord` implementation treating `None` as less than `Some(_)`.

  ### Problem

  The derived `Ord` implementation for `ChainPosition::Unconfirmed` caused incorrect transaction ordering:
  - `Unconfirmed { first_seen: None, last_seen: None }` (never in mempool)
  - Would appear **before** `Unconfirmed { first_seen: Some(10), last_seen: Some(10) }` (seen in mempool)

  This resulted in a confusing wallet display where transactions never broadcast would appear before pending mempool transactions.

  ### Solution

  Implemented manual `Ord` and `PartialOrd` traits for `ChainPosition` with the correct ordering:

  1. **Confirmed transactions** (earliest first by block height)
  2. **Unconfirmed transactions seen in mempool** (ordered by `first_seen`)
  3. **Unconfirmed transactions never seen** (these come last)

  Additional ordering rules:
  - At the same confirmation height, transitive confirmations come before direct confirmations (as they may actually be confirmed earlier)
  - Tie-breaking uses transaction IDs for deterministic ordering

  ## Changelog notice

  ### Fixed
  - Fixed `ChainPosition` ordering so unconfirmed transactions never seen in mempool appear last instead of first

  ### Changed
  - `ChainPosition`, `CanonicalTx`, and `FullTxOut` now require `A: Ord` for their `Ord` implementations
  - Simplified `FullTxOut` ordering to only use essential fields (chain_position, outpoint, spent_by)

  ## Checklists

  ### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  ### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

Top commit has no ACKs.

Tree-SHA512: a8f4f2c73391ea5425870f365ca2b66b48ac9466211e82af6ed832673f18fec7812cf99abed7b1620e8d26ed47ace6b3b75902e50880b411d96946dfb82c6dad