Fix #573: Emit success event from accrue()

[badmusdare4]

��## Summary

• Emit a success event from BlendAdapter::accrue() carrying the previous and new cached otal_assets() values. This makes �ccrue() genuinely observable for keepers.
• Added a unit test to verify the event is emitted with expected payload.

Test plan

• cargo test passes locally
• Asserted event fires with expected values

Closes #573

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[collinsezedike]

@badmusdare4 the fix itself is correct, verified prev is read before the storage write and the test's rate math checks out. A few things need fixing before this can merge:

• PR Title/Commit Messages fail: the title (Fix #573: Emit success event from accrue()) doesn't follow CONTRIBUTING.md's commit convention (type(scope): subject), it becomes the squash commit message.
• Soroban Contract Tests fails on cargo fmt --all -- --check, two lines in the new test exceed the line width. Run cargo fmt --all before pushing.
• The PR body has corrupted control characters mid-word (otal_assets(), and a stray character before ccrue()), looks like a paste-encoding issue. Please re-paste it cleanly.