extraction: add ITBTCVaultMigrationDebt interface (compile-fix)

Bridge.sol on this branch imports
`../vault/ITBTCVaultMigrationDebt.sol` (and references the
interface's `hasOutstandingMigrationDebt.selector` via a
fail-open staticcall) but the interface file was not extracted
from the tlabs-xyz/tbtc umbrella into this canonical mirror.
Both `contracts-format` and `contracts-slither` fail at
compile-time with `HH404: File ... ITBTCVaultMigrationDebt.sol
not found`.

This commit pulls the interface file from umbrella as-is per the
"import as-is first" rule. The 94-line file is a pure interface
(events + function selectors, no external imports) so it drops
in cleanly.

Behavior: Bridge.sol's `hasOutstandingMigrationDebt.selector`
staticcall is intentionally fail-open — if the vault doesn't
implement the interface (which canonical TBTCVault doesn't yet),
the guard is skipped. The full vault-side migration-debt
implementation lands in a separate extraction PR (alongside
ITBTCVaultMigrationSweepHook, ITBTCVaultMigrationSweepNotifier,
TBTCMigrationDebtOperations).

This PR remains [DO NOT MERGE - hard drain] per its title —
post-drain operational prerequisites haven't been met. CI green
just means the PR is review-ready for when the drain completes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mswilkison

solidity/contracts/vault/ITBTCVaultMigrationDebt.sol

@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+pragma solidity 0.8.17;
+
+/// @notice Interface for TBTCVault migration debt accounting.
+interface ITBTCVaultMigrationDebt {
+    event MigrationDebtRegistered(address indexed revealer, uint256 amount);
+    event MigrationDebtRepaid(address indexed revealer, uint256 remaining);
+    event MigrationDebtCleared(address indexed revealer, uint256 amount);
+    event MigrationRevealerSet(address indexed revealer, bool allowed);
+
+    /// @notice Registers the expected total migration debt for a revealer.
+    ///         Must be called before revealing migration-tagged deposits.
+    ///         The revealer cannot reveal migration deposits without an
+    ///         outstanding debt registration (enforced by
+    ///         `canRevealMigration`).
+    /// @dev The `amount` parameter should represent the expected total BTC
+    ///      value of migration deposits for this revealer, converted to
+    ///      1e18 Ethereum precision.
+    ///
+    ///      Amount convention trade-off: the caller must decide whether to
+    ///      register the gross (pre-fee) deposit value or an estimated net
+    ///      (post-fee) value:
+    ///
+    ///      Gross registration (registering the full pre-fee deposit
+    ///      amount): conservative approach. After the Bridge sweeps the
+    ///      deposit, it deducts a treasury fee per deposit
+    ///      (`depositAmount / depositTreasuryFeeDivisor`) and a share of
+    ///      the Bitcoin sweep transaction miner fee
+    ///      (`depositTxFeeIncurred`). The post-fee amount reaching the
+    ///      vault is less than the registered debt, leaving a residual
+    ///      positive debt. Migration lifecycle completion requires an
+    ///      owner-initiated `notifyMigrationSweep` call to transition
+    ///      the reserve to MIGRATED state. This approach avoids excess
+    ///      tBTC minting.
+    ///
+    ///      Net registration (estimating post-fee value): if the fee
+    ///      estimate matches actual deductions, debt reaches zero
+    ///      automatically on sweep and triggers the migration completion
+    ///      callback. However, the Bitcoin miner fee component is
+    ///      unknowable at registration time since it depends on the
+    ///      sweep batch composition and Bitcoin fee market conditions.
+    ///      Underestimating fees causes excess tBTC minting;
+    ///      overestimating fees leaves residual debt requiring owner
+    ///      intervention.
+    ///
+    ///      Registration is one-time: reverts if the revealer already has
+    ///      outstanding debt.
+    /// @param revealer The address that will reveal migration-tagged
+    ///        deposits on the Bridge.
+    /// @param amount Expected migration debt in 1e18 Ethereum precision
+    ///        (satoshi values are scaled by SATOSHI_MULTIPLIER = 10^10).
+    function registerMigrationDebt(address revealer, uint256 amount) external;
+
+    function migrationDebt(address revealer) external view returns (uint256);
+
+    /// @notice Clears any remaining migration debt for `revealer`.
+    /// @dev Intended for the over-registration residual-debt case once the
+    ///      migration lifecycle has been completed out-of-band and no further
+    ///      migration deposits should be revealed for this address.
+    ///      Implementations should zero the debt, update aggregate debt
+    ///      accounting, and disable migration reveals for `revealer`.
+    /// @param revealer The revealer whose residual migration debt should be
+    ///        cleared.
+    function clearMigrationDebt(address revealer) external;
+
+    function setMigrationRevealer(address revealer, bool allowed) external;
+
+    function isMigrationRevealer(address revealer)
+        external
+        view
+        returns (bool);
+
+    /// @notice Returns whether the given revealer is allowed to reveal
+    ///         migration-tagged deposits.
+    /// @dev Returns true only when both `isMigrationRevealer[revealer]`
+    ///      is set and `migrationDebt[revealer] > 0`. This ensures a
+    ///      revealer cannot reveal migration deposits without a
+    ///      corresponding debt registration, and cannot reveal beyond
+    ///      the registered capacity once debt is fully repaid.
+    /// @param revealer Address to check migration reveal eligibility.
+    /// @return True if the revealer can reveal migration deposits.
+    function canRevealMigration(address revealer) external view returns (bool);
+
+    /// @notice Returns whether this vault has any outstanding migration debt
+    ///         across all revealers.
+    /// @dev Implementations should maintain an internal counter that is
+    ///      incremented when migration debt is registered and decremented
+    ///      when a revealer's debt reaches zero, whether via repayment or an
+    ///      owner-authorized residual debt clear-out. This provides an O(1)
+    ///      aggregate query without enumerating individual revealers.
+    /// @return True if at least one revealer has nonzero migration debt.
+    function hasOutstandingMigrationDebt() external view returns (bool);
+}