loopd: support more HTLC recovery modes, stateless and cooperative

[starius]

Allow sweephtlc to reconstruct protocol-11 Loop Out HTLCs when the local swap record is unavailable but the public swap data and preimage can be recovered.

Prefer signing by public key and use a bounded family-99 key scan only when direct signing fails or produces an invalid witness.

Also add cooperative sweep as an option applicable in both stateful and stateless modes. In stateless mode it also requires providing payment address.

So now we have a matrix.

State	Spend	LND key path	Mode note
Stateful	Unilateral	Stored locator	Loads the preimage, contract, and locator from Loop DB; spends the Taproot script success path.
Stateful	Cooperative	Stored locator	Loads the contract, locator, and invoice payment-address proof from Loop DB; uses the MuSig2 Taproot key path without revealing the preimage to the server.
Stateless	Unilateral	Public key	Reconstructs the contract without Loop DB and asks LND to sign directly with client_pubkey; no key scan occurs.
Stateless	Unilateral	Scan fallback	Reconstructs the contract without Loop DB; an unknown public-key signing result must fail local witness verification, then family-99 scanning recovers the locator.
Stateless	Cooperative	Locator scan	Reconstructs the contract without Loop DB, scans family 99 for the locator required by LND's MuSig2 API, and supplies the invoice payment_addr as server ownership proof.

I tested all the modes locally.

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[bhandras]

LGTM 🎉