aliasmgr: Allow persisting manually added alias scids

[GeorgeTsagk]

Description

Previously, all aliases were removed from the aliasToBase map when a channel reached 6 confirmations via DeleteSixConfs, making them unfindable via FindBaseSCID. This was problematic for manually-added aliases (e.g., via RPC) that users expected to persist. This PR adds a 1-byte flag field to the alias storage format, extending entries from 8 bytes (base SCID) to 9 bytes (base SCID + flag). This flag simply marks the persistence intention for that alias entry.

Now when the user manually adds a local channel alias they may signal the intention to persist the alias by re-using the optional argument WithBaseLookup (the only call site at the moment is the RPC endpoint XAddLocalChanAliases.

Tests have been updated to cover the behavior of old & new entries under the new format with the persistence flag.

[gemini-code-assist[bot]]

Summary of Changes

Hello @GeorgeTsagk, I'm Gemini Code Assist[^1]! 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 enhances the alias manager functionality by introducing the ability to persist manually added channel aliases. Previously, such aliases would be removed once a channel reached six confirmations, leading to unexpected behavior for users. By adding a persistence flag to the alias storage and modifying the alias management logic, this change ensures that aliases explicitly marked for persistence remain accessible, improving the reliability and usability of manual alias management, especially for RPC-driven operations.

Highlights

• Alias Persistence: Introduced a persistence flag for channel aliases, allowing manually added aliases to remain findable after channel confirmation.
• Storage Format Update: Modified the alias storage format to include a 1-byte flag, extending entries from 8 to 9 bytes while maintaining backward compatibility.
• WithBaseLookup Option: The WithBaseLookup option for AddLocalAlias now explicitly marks an alias as persistent, ensuring it survives channel confirmation.
• Manager Logic Updates: Updated alias manager logic to respect the persistence flag during channel confirmation and manager reloads, preventing unintended deletion of persistent aliases.
• Comprehensive Testing: Added comprehensive tests to validate the behavior of persistent aliases, non-persistent aliases, and backward compatibility with old alias formats.

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[lightninglabs-deploy]

@georgetsagk, remember to re-request review from reviewers when ready

[GeorgeTsagk]

Do we need to allow RPC callers to opt out of this behavior?

I'd expect the intuitive thing to be that a manually added entry requires manual deletion. Opt-ing out has some weird side effects like the fact that if you add the alias over a confirmed channel then it won't appear again after a restart (as by default aliases are not reloaded for confirmed channels).