channeldb+migration: add type-prefixed waiting proof records

[ellemouton]

Summary

Part of #10293 (Gossip 1.75 epic).

The waiting proof store holds half-proofs (AnnounceSignatures) while
we wait for the remote peer's matching half. To support gossip v2
(taproot channels), the store needs to handle both AnnounceSignatures1
and AnnounceSignatures2 payloads.

This PR adds a 1-byte type discriminator to every record so Decode
knows which wire message to parse, and introduces the codec machinery
for both proof types.

Record format

Before (legacy):

┌──────────┬───────────────────────────┐
│ isRemote │  AnnounceSignatures1      │
│ (1 byte) │  (wire payload)           │
└──────────┴───────────────────────────┘

After:

┌──────────┬──────────┬───────────────────────────┐
│   type   │ isRemote │  AnnounceSignatures*      │
│ (1 byte) │ (1 byte) │  (wire payload)           │
└──────────┴──────────┴───────────────────────────┘

Type byte values

Value	Constant	Payload	Channels
0x00	WaitingProofTypeV1	AnnounceSignatures1	P2WSH
0x01	WaitingProofTypeV2	AnnounceSignatures2 + optional aggregate MuSig2 nonce	Taproot

V2 payload detail

V2 records carry an optional aggregate nonce needed to reconstruct the
final MuSig2 signature:

┌───────────────┬──────────────────────┬─────────────────────┐
│ aggNonce flag  │ aggNonce (33 bytes)  │ AnnounceSignatures2 │
│ (1 byte bool)  │ (if flag set)        │ (TLV payload)       │
└───────────────┴──────────────────────┴─────────────────────┘

Commits

1. channeldb: add type-prefix to waiting proof store records

   • Adds mandatory migration 35 that rewrites all existing records with
     a 0x00 type prefix
   • Updates WaitingProof.Encode/Decode to write/expect the prefix
   • Migration + codec change kept atomic so no intermediate revision can
     encounter unmigrated records

2. channeldb: add V2 (taproot) waiting proof codec support

   • Introduces WaitingProofInner interface with V1WaitingProof and
     V2WaitingProof implementations
   • Encode/Decode now dispatch on the type byte
   • Gossiper updated with a V1 type assertion (full V2 gossiper
     integration will follow separately)
   • No live code path creates V2 proofs yet — this only lands codec
     • storage readiness

Test plan

• Migration happy path: legacy records rewritten with type prefix
• Migration key-mismatch: corruption detected and rejected
• V1 encode prefix assertion
• Unknown type decode rejection
• V2 round-trip: with and without aggregate nonce
• V2 store integration: add/get/remove through WaitingProofStore
• make lint passes on both commits

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces foundational changes to the channeldb's WaitingProofStore to support the upcoming gossip v2 and taproot channels. It achieves this by implementing a new database migration that prefixes existing waiting proof records with a type byte, and by refactoring the WaitingProof structure to be type-agnostic through an interface. This allows the system to store and decode both legacy (V1) and new (V2, taproot-compatible) AnnounceSignatures payloads, preparing the database for future taproot channel announcements without immediately enabling their live creation or full gossiper integration.

Highlights

• Database Migration 35: Introduced a mandatory database migration to prefix all existing WaitingProof records with a 0x00 type byte, signifying WaitingProofTypeV1.
• Type-Prefixed Waiting Proofs: Modified the WaitingProof record format to include a 1-byte type discriminator, enabling the storage of different AnnounceSignatures versions.
• Flexible Waiting Proof Structure: Refactored WaitingProof to embed a WaitingProofInner interface, allowing for distinct implementations like V1WaitingProof (for AnnounceSignatures1) and V2WaitingProof (for AnnounceSignatures2).
• Taproot Channel Readiness: Added V2WaitingProof to support AnnounceSignatures2 payloads, including an optional aggregate MuSig2 nonce, laying the groundwork for taproot channel gossip.
• Gossiper Adaptation: Updated the AuthenticatedGossiper to handle the new WaitingProof structure, explicitly asserting V1 proof types for current operations, with V2 integration planned for later.

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Changelog

• channeldb/db.go
  • Imported migration35 package.
  • Registered MigrateWaitingProofStore as a mandatory database migration.
• channeldb/log.go
  • Imported migration35 package.
  • Initialized logger for migration35.
• channeldb/migration35/log.go
  • Added logging setup for the migration35 package.
• channeldb/migration35/migration.go
  • Implemented database migration 35 to prefix existing waiting proof records with a V1 type byte.
• channeldb/migration35/migration_test.go
  • Added unit tests for the MigrateWaitingProofStore database migration.
• channeldb/waitingproof.go
  • Refactored WaitingProof to support multiple types (V1 and V2) using an interface.
  • Added V1WaitingProof and V2WaitingProof implementations.
  • Updated encoding/decoding logic to include a type prefix and dispatch based on it.
• channeldb/waitingproof_test.go
  • Added tests for waiting proof encoding/decoding with type prefixes and V2 proof functionality.
• discovery/gossiper.go
  • Updated handleAnnSig to explicitly cast waiting proofs to V1 type, preparing for V2 integration.

Activity

• The author, ellemouton, has implemented a new database migration (35) to update the WaitingProofStore format.
• The WaitingProof structure has been refactored to support multiple versions of AnnounceSignatures through an interface.
• New V1WaitingProof and V2WaitingProof types have been introduced, with V2WaitingProof supporting an optional aggregate MuSig2 nonce for taproot channels.
• The AuthenticatedGossiper has been updated to handle the new WaitingProof structure, with a temporary assertion for V1 proofs.
• Comprehensive tests have been added for the migration, encoding/decoding of V1/V2 proofs, and store integration.

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[ellemouton]

@codex review this

[ellemouton]

@claude review this

[gemini-code-assist]

Code Review

This pull request introduces a type prefix to WaitingProof records to support different gossip versions, specifically for upcoming Taproot channel support. It includes a database migration to update existing records to the new format. The WaitingProof struct is refactored to use an interface, making it extensible for future proof types. The changes are well-tested, including migration tests and unit tests for the new V2 proof logic. My review focuses on adherence to the repository's style guide regarding code documentation and formatting. I've suggested adding more in-body comments to explain the intention of code blocks, as recommended by the style guide.