feat: vault-balance-sync — closes #SC-085

[alade-dev]

Closes #SC-085 — Vault Balance Sync via Treasury Callback (#366)

Summary

Implements sync_vault_balance(call_id) pulling vault_balance from call_registry via cross-contract CallRegistryClient::get_call and caching in Map<u64, i128> (SyncedVaultBalance) for analytics, with get_synced_vault(call_id) view.

Changes

• packages/contracts-stellar/treasury/src/treasury.rs: add DataKey::SyncedVaultBalance(u64), call_registry_iface with full Call mirror (13 fields) for cross-contract deserialization, sync_vault_balance (permissionless, TTL bump, VaultBalanceSynced event), get_synced_vault view
• packages/contracts-stellar/treasury/src/test.rs: MockCallRegistry with full MockCall mirror, 6 tests (sync matches registry, missing reverts, cached view correct, permissionless, idempotent, zero, updated registry)

Acceptance Criteria

• Sync matches registry value
• Missing call reverts (Call does not exist)
• Cached view correct via get_synced_vault

Checks

cargo test --workspace
# 139 tests: 76 call_registry + 19 outcome_manager + 38 treasury + 6 liquidity — all pass
cargo clippy -p treasury -- -D warnings  # pass
cargo fmt --check  # pass
# soroban CLI not installed — wasm build skipped (treasury crate-type cdylib OK via cargo test)

Notes

• cargo clippy --workspace -- -D warnings fails on pre-existing call_registry warnings (unnecessary_cast, manual_range_contains) — not introduced by this PR; treasury alone passes clean
• governance::errors::ContractError has 50 variants (at limit for #[contracterror] macro); missing-call case propagates registry's Call does not exist panic rather than adding a new error code