Added splicing plumbing

[ihordiachenko]

Rationale

This PR prepares Greenlight for splicing support without waiting for the VLS side of things. This establishes the CLN API, binding, persistence, and policy boundaries needed for later signer integration.

What Changed

• Added low-level CLN splice RPC plumbing and v26.06 bindings.
• Persisted splice sessions, wallet PSBT context, authorization metadata, and outpoint indexes.
• Captured relevant CLN response facts and linked PSBTs to splice sessions.
• Added a signer policy resolver that validates known invariants and fails closed at the missing VLS proof boundary.
• Added trace and regtest coverage for policy decisions and signer rejections.

Trade-offs

• Splices are exposed via experimental-splicing Cargo feature but cannot complete until VLS provides the required signing proofs.

Handling PBSTs

Greenlight links each splice PSBT to its splice session during SpliceInit and records whether funding and signing were authorized. The signer rejects it unless the link and signing approval are present. Unrelated PSBTs remain outside splice handling and follow the existing signer policy.