fix(fee_collector): add lifetime_withdrawn tracking and reconciliation view

[DavidAkere204]

• Add KEY_WITHDRAWN persistent storage key per token
• Update withdraw() to increment the lifetime-withdrawn counter on every successful withdrawal (parallel to KEY_TOTAL / lifetime-collected)
• Add get_total_withdrawn(token) view function
• Add get_expected_balance(token) view function: expected = get_total_collected(token) - get_total_withdrawn(token) Diffing this against get_balance(token) surfaces any drift between reported accounting and actual token holdings
• Update module doc comment to clearly state that get_total_collected is a reported/claimed figure, not independently verified against real token movement, and document the reconciliation invariant
• Add test_reconciliation_detects_divergence covering:
  • normal collect + withdraw produces zero drift
  • over-reported collect_fee (50 reported vs 30 received) produces a detectable 20-token drift via get_expected_balance - get_balance

Closes #41