feat(indexer): sync outgoing state

[m-kus]

TL;DR

Added outgoing state synchronization for privacy pools with improved channel key handling.

What changed?

• Increased COST_OUTGOING_CHANNEL_INFO from 2 to 3 to account for public key retrieval
• Enhanced OutgoingChannel struct to include a channel_key derived from sender and recipient identities
• Added channel key registration in the discovery cursor
• Created a new outgoing_state module that orchestrates the full outgoing sync workflow:
  • Implements hierarchical discovery (channels → subchannels → note indices)
  • Parallelizes channel and subchannel processing using tokio::task::JoinSet
  • Provides cursor-based pagination with pruning of fully-discovered elements
• Added comprehensive tests for full discovery and pagination scenarios

How to test?

Run the new tests in the outgoing_state module:

• test_sync_outgoing_state_full_discovery: Verifies complete discovery with a generous budget
• test_sync_outgoing_state_pagination: Tests pagination across multiple calls with limited budgets

Why make this change?

This change completes the outgoing state synchronization workflow, enabling efficient discovery of outgoing channels, subchannels, and note indices. The addition of channel key handling improves the security model, while the parallelized processing enhances performance. The pagination support with cursor pruning ensures efficient resource usage during discovery operations.

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This change is 

[m-kus]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(indexer): e2e discovery flow #440
• feat(indexer): preflight endpoint, sync with abstract discovery provider interface #439
• feat(indexer): outgoing state sync endpoint #438
• feat(indexer): batch state sync #435
• feat(indexer): sync outgoing state #433 👈 (View in Graphite)
• feat(indexer): discover outgoing channels #430
• feat(indexer): find last used note index #429
• feat(indexer): outgoing channel primitives #424
• feat(indexer): filter out spent notes, use secret felt #422
• feat: scaffold e2e testing harness, move devnet dump generation to a separate script #421
• feat(indexer): sync code and spec, update guidelines and todos #414
• feat(indexer): add incoming state sync endpoint #413
• feat(indexer): api error types #412
• refactor(indexer): common helpers for spawning devnet/indexer #411
• feat(indexer): incoming state sync orchestration #410
• feat(indexer): cursor pagination for discovery #408
• refactor(indexer): reorganize discovery-core, prepare for state sync #407 : 1 other dependent PR (#453 )
• feat(ci): speed up rust workflow #415
• main

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