Support abstract-level v3 APIs

[germanoeich]

Summary

Adds the abstract-level v3 API surface needed by rave-level / Foundry:

• updates the test suite for abstract-level v3 behavior
• adds RPC support for has() and hasMany()
• adds explicit snapshot support over RPC
• wires snapshots through get, getMany, has, hasMany, iterators, keys, values, and clear
• ensures retry mode correctly reports snapshots as unsupported
• adds opt-in getSync support:
  • forwarded/local DBs call the underlying _getSync()
  • remote users can provide a custom sync handler
  • plain remote guests still reject with LEVEL_NOT_SUPPORTED

Notes

getSync is intentionally not implemented as a generic async RPC wait inside many-level. Blocking the event loop while waiting on the same async stream can deadlock. many-level now exposes the minimal hook needed for consumers like rave-level to provide their own sync transport strategy.

Related PR: ForgeVTT/rave-level#5

[oneirocosm]

I'm a little confused by line 39 of the guest.js file

    const hasGetSync = getSync === true || typeof getSync === 'function'

this implies that getSync can be passed as a boolean, but later in the file, get sync is set with this

    this[kGetSync] = typeof getSync === 'function' ? getSync : null

To me, this implies that if getSync is passed in as true, the function itself will remain null. I imagine there could be a reason for this if the class was extended, but I want to double check instead of just speculating.

[germanoeich]

@oneirocosm good catch, that's the cursed nature of having a local leader and remote follower distinction in the same process.

getSync: true is an opt-in for the local DB path (leader). In that case _getSync() first checks this[kDb] and delegates to the underlying database’s _getSync(), so there is no custom handler to store in kGetSync.

getSync: function is the remote/custom-handler path (follower). That is what consumers like rave-level use when a follower needs to provide its own synchronous transport strategy.

So yes, if getSync is passed as true, this[kGetSync] remains null. Support is advertised for the forwarded DB case, not because there is a standalone remote sync handler.