feat(whitelist): store entries as keyed persistent flags (#1220)

[Nuruddeen61]

Closes #1220

Rebuilt on top of current main to address review feedback on #1259.

Changes

• Whitelisted(epoch, address) -> slot and WhitelistSlot(epoch, slot) -> address in persistent storage, so is_whitelisted is an O(1) keyed lookup
• Instance storage keeps scalars only: WhitelistEpoch, WhitelistCount, WhitelistMigrated, WhitelistMigrationCursor
• add_address / remove_address: O(1) insert and swap-remove with TTL bumps
• clear_all: O(1) epoch bump
• migrate_legacy: batched (MAX_MIGRATION_BATCH = 100), idempotent and cursor-based; mutations return MigrationPending until it finishes
• get_whitelist_page and whitelist_count added for large lists

Review feedback addressed

• All 21 existing unit tests are kept (the error-code test is extended with codes 53 and 54); 7 new tests added (swap-remove, epoch clear, paging, legacy fallback reads, batched and idempotent migration, mutations blocked during migration, non-admin migration).
• New error MigrationPending = 54 (53 is already AdminTransferCancelled)
• Events, cancel_admin_transfer and cool-off behavior are preserved
• Rebased on current main, no conflicts

Also fixes a compile error on main: cancel_admin_transfer referenced an undefined
new_admin (the admin_accepted event block belongs in accept_admin); moved it back.

[Nuruddeen61]

CI is currently red on main independent of this PR:

• cargo fmt --check fails in contracts/vault (test_simulate_parity.rs, test_timelock.rs)
• the workspace fails to compile in contracts/settlement (batch.rs / lib.rs: two SettlementError
  types and a batch_settle signature mismatch). fix(settlement,distribute): reconcile TotalReceived across all credit paths and fix batch_distribute compile #1362 looks like it addresses this.
  This PR only touches contracts/whitelist (lib.rs, errors.rs). I'll rebase once main is green
  and report the whitelist results then.