PSwap: Collapse NoteObserver into the existing OnNoteReceived abstraction

[VAIBHAVJINDAL3012]

Context

Follow-up from PR #2231 review (thread). Quoting @igamigo's approval review:

I'd want to remove the note observer trait and collapse it into the existing OnNoteReceived. The setup feels a bit more complex than it needs to be right now. The alternative could be to remove the traits and hardcode the logic, and try to do a separate PR for making it more generic (e.g., as part of #2209). This way we also reduce the diff.

Problem

The PSWAP tracking work introduced a new NoteObserver trait (crates/rust-client/src/sync/note_observer.rs:16) alongside the pre-existing OnNoteReceived screening callback. They overlap: OnNoteReceived filters notes, while NoteObserver collects filtered notes and applies follow-up logic. This is one abstraction more than necessary.

Proposed work

• Fold NoteObserver's responsibilities into OnNoteReceived (or a minimal extension of it), removing the standalone trait.
• Re-express PswapChainObserver on top of the unified abstraction.
• Alternatively, if a clean unification isn't possible now, hardcode the PSWAP logic and pursue the generic version separately under Feature to subscribe based on attachment and tag, instead of just tag. #2209 — whichever reduces surface area.

Acceptance criteria

• NoteObserver trait removed; PSWAP discovery runs through OnNoteReceived.
• No behavioral change to PSWAP lineage tracking (existing pswap_* tests stay green).

[VAIBHAVJINDAL3012]

Scoping this before I pick it up.

Right now OnNoteReceived (sync/state_sync.rs:109) is just a per-note callback that returns a verdict — Commit / Insert / Discard. There's one screener per sync and it never writes to the store itself; the verdict only feeds the NoteUpdateTracker.

NoteObserver does two things the screener can't:

• it gets the note's attachments and collects the ones it cares about (PSWAP filters by attachment scheme), and
• it has an apply() hook that runs once after the sync, to correlate and persist what it collected.

So to fold it into OnNoteReceived, the trait needs to change in two ways:

1. pass the note's attachments into the callback, so a handler can collect the notes it wants, and
2. add an apply()-style hook so the handler can act on the collected notes after the sync finishes.

Then PswapChainObserver is just another OnNoteReceived and we drop the separate trait.

Does this direction look right?
cc : @igamigo @SantiagoPittella

[igamigo]

So to fold it into OnNoteReceived, the trait needs to change in two ways:

1. pass the note's attachments into the callback, so a handler can collect the notes it wants, and
2. add an apply()-style hook so the handler can act on the collected notes after the sync finishes.

Agreed with this. Regarding 1, I think we want to pass attachments as part of CommittedNote directly instead of as a separate parameter. Or rather, conceptually that should be the case though the refactor to do this may not be super trivial (because the attachment data comes separately from the committed note data). If it's too complicated this refactor can be left for #2118

[VAIBHAVJINDAL3012]

Yes, ideally it should be a part of CommitNote.
I can check if we can pass it separately for now, and if future refactor it in 2118?

Does that sound good?