Migrate from levelup/subleveldown to classic-level/sublevel

[axel-krapotke]

Background

The current LevelDB stack uses the deprecated levelup + leveldown + subleveldown packages. These have been superseded by classic-level (or level) with built-in .sublevel() support.

Current State

The persistent command queue (queue.mjs) uses the levelup API:

• createReadStream() for iterating entries
• Callback-based batch() and del()
• subleveldown for namespace isolation

Target State

Migrate to the abstract-level API:

• iterator().all() instead of createReadStream()
• Promise-based batch(), del(), get(), put()
• Built-in .sublevel() instead of subleveldown

Notes

• The on-disk format is identical between old and new APIs (same LevelDB C++ engine). User data requires no migration.
• This should be coordinated with the corresponding migration in ODINv2 (see syncpoint/ODINv2 issue).
• Tests currently use memdown + levelup + subleveldown — migrate to memory-level.