fix(wallet): allow fee bump without opt-in RBF signaling

[alienx5499]

Description

Fixes #65.

Wallet::build_fee_bump previously refused to build a replacement when no input signaled opt-in RBF (nSequence ≤ 0xFFFFFFFD). That guard is removed so the wallet can construct a fee-bump transaction even if the original did not opt in. Whether a replacement is accepted is a mempool / relay policy concern (e.g. default full-RBF in recent Bitcoin Core), not something this API can enforce by requiring a specific nSequence on the prior tx.

Also removes BuildFeeBumpError::IrreplaceableTransaction, since that path is no longer reachable.

Notes to the reviewers

• API: BuildFeeBumpError::IrreplaceableTransaction is removed (breaking public enum). If you prefer a deprecation cycle for the variant only, say so and we can adjust.
• Semantics: Callers could previously rely on an error when the original tx did not signal opt-in RBF; they must not assume that anymore.
• Tests: tests/build_fee_bump.rs — the old test expected failure for non–opt-in-RBF; it now asserts a successful fee bump with a higher fee rate.

Changelog notice

fix(wallet): allow build_fee_bump without opt-in RBF on the original tx; remove IrreplaceableTransaction (#65)

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.04%. Comparing base (fb7681a) to head (db7feac).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #430   +/-   ##
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  Coverage   80.04%   80.04%           
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  Files          24       24           
  Lines        5336     5326   -10     
  Branches      242      241    -1     
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- Hits         4271     4263    -8     
+ Misses        987      985    -2     
  Partials       78       78           

Flag	Coverage Δ
rust	80.04% <ø> (+<0.01%)	⬆️

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