fix: preserve rbf replacement fee rates

[pwltr]

Related: synonymdev/bitkit-ios#485

Companion: iOS #879.

This PR preserves an outgoing RBF replacement's selected fee rate when activity arrives or is refreshed, so consumer apps can show the correct confirmation estimate.

Description

• Adds atomic RBF recording so the replacement's rate is retained as pending metadata or applied immediately when its activity already exists, without resurrecting a removed original.
• Adds wallet-scoped, fee-preserving onchain snapshot upserts so late sync and transaction events cannot overwrite the recorded rate or create a duplicate activity under a different payment ID.
• Preserves existing tags and metadata, rolls back partial database changes on failure, and leaves explicit generic fee-rate corrections available.
• Exposes both operations through UniFFI and includes the generated Swift interface and FFI declarations.

Out of Scope

• Activity history: correcting fee rates already stored incorrectly before this fix.
• Platform SDK publication, version/checksum updates, Android consumer adoption, and incoming CPFP behavior.

Preview

N/A — shared persistence APIs; no UI changes.

QA Notes

Manual Tests

• Consumer app → send an unconfirmed onchain transaction at a low custom fee, boost via RBF, and open its replacement → the confirmation estimate matches the boosted rate and the actual replacement fee is retained.
• Consumer app → refresh, relaunch, and confirm the replacement → its recorded fee rate remains correct.

The author verified the fix in the iOS simulator using a local SDK built on the apps' existing 0.5.18-compatible base. The draft is rebased onto current core master; the activity implementation and tests are identical to that tested snapshot. Integration with the full current-master SDK must be checked when publishing/updating the consumer dependency.

Automated Checks

• added rbf_tests.rs — ten regressions cover replacement arrival order, persistence across database reopen, stale snapshots, wallet isolation, invalid inputs, metadata preservation, and atomic rollback/retry.
• ran the activity suite on the 0.5.18-compatible fix snapshot — 206 passed, zero failures; focused RBF regressions also passed.
• ran Clippy on that snapshot — existing warnings only; none in the changed activity implementation or RBF test file.
• ran a simulator-only SDK build and the iOS consumer regression suites — all 32 iOS tests passed, including eight RBF tests and existing activity/hardware-wallet/transfer coverage.
• ran Rust formatting and generated Swift interface typechecking after rebasing onto master — passed.