feat: add reserve lifecycle events

[Cybermaxi7]

PR Title

feat: Add reserve lifecycle events for sweep and expiration
Closes #30

Description

This PR adds event visibility for the base reserve (1 XLM) reclamation process during account sweep and expiration operations.

Changes

New Events:

• Added ReserveReclaimed event with:
  • destination: Address - where the reserve is returned
  • amount: i128 - reserve amount in stroops (1 XLM = 1,000,000,000 stroops)

Extended Events:

• AccountExpired now includes reserve_amount: i128 field

Implementation:

• sweep() now emits ReserveReclaimed after successful sweep
• expire() now emits both AccountExpired (with reserve_amount) and ReserveReclaimed
• Reserve amount is a constant 1 XLM base reserve per Stellar protocol

Tests:

• Added test_sweep_emits_reserve_reclaimed_event
• Added test_sweep_multiple_assets_with_reserve_event

Cleanup:

• Deleted placeholder contracts/reserve_contract/reserve.md

Acceptance Criteria

• Events emitted on sweep reclaim
• Events emitted on expiration reclaim
• Covered by tests
• No documentation-only implementation remains
• All reserve functionality exists in code
• Workspace builds and tests pass

Safety Considerations

This change only adds event emission and does not modify any balance transfer logic or state management. The reserve amount is a constant (1 XLM) and is emitted for off-chain indexing/monitoring purposes only.