feat: add transaction history methods to UniFFI bridge

[xdustinface]

Summary

• Add get_transactions(limit, offset) method for paginated transaction history
• Add get_transaction(txid) for single transaction lookup
• Add get_transaction_count() method
• Stub implementations returning empty data (wallet integration later)
• Bridge tests for new methods

Closes #40

Test plan

• get_transactions returns empty vec (stub)
• get_transaction returns None for unknown txid
• get_transaction_count returns 0 (stub)

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.57%. Comparing base (8ae458c) to head (a565aa2).
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Additional details and impacted files

@@              Coverage Diff              @@
##           v0.42-dev      #41      +/-   ##
=============================================
- Coverage      72.83%   67.57%   -5.26%     
=============================================
  Files            287      316      +29     
  Lines          57486    67742   +10256     
=============================================
+ Hits           41870    45778    +3908     
- Misses         15616    21964    +6348     

Flag	Coverage Δ
core	75.14% <ø> (ø)
ffi	37.82% <ø> (?)
rpc	19.92% <ø> (ø)
spv	84.34% <100.00%> (+0.04%)	⬆️
wallet	66.40% <ø> (ø)

Files with missing lines	Coverage Δ
dash-spv/src/bridge/mod.rs	90.36% <100.00%> (+0.55%)	⬆️

... and 31 files with indirect coverage changes

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