Wire `BatchNoteTree` into batch & block construction

[mmagician]

Background / motivation

• The batch kernel should output the root of a BatchNoteTree built over the batch's output notes,
• The block kernel then aggregates the per-batch BatchNoteTrees into a single BlockNoteTree,

Before the kernel can output (and later verify) BATCH_NOTE_TREE_ROOT in MASM, the Rust side must actually construct the BatchNoteTree during batch construction.

Batch

What already exists:

• BatchNoteTree is a wrapper over SimpleSmt<BATCH_NOTE_TREE_DEPTH>. It is built via BatchNoteTree::with_contiguous_leaves, which packs note IDs at contiguous leaf indices starting at 0; each leaf value is the note ID (hash(note_details_commitment || note_metadata_commitment)).

The gap:

• BatchNoteTree is never constructed during batch building. ProposedBatch::new -> new_batch_inner uses NoteTracker to compute and erase output notes, then stores them only as a flat Vec<OutputNote> exposed via ProposedBatch::output_notes(). No BatchNoteTree and no batch-note-tree root is computed or stored on ProposedBatch.

Proposed change:

• Construct the BatchNoteTree during batch construction. In ProposedBatch::new_batch_inner, after the NoteTracker finalizes and the final (non-erased) output_notes: Vec<OutputNote> is computed, build a BatchNoteTree from those output notes' headers and store it on ProposedBatch. Add an accessor (e.g. batch_note_tree()) and include it in into_parts.

Block

What already exists:

• BlockNoteTree::insert_batch_note_subtree(batch_idx, batch_note_tree): inserts a BatchNoteTree as a subtree of the BlockNoteTree (depth 16) at batch_idx via SimpleSmt::set_subtree.

The gap:

• The above BlockNoteTree::insert_batch_note_subtree is never used. At the block level, ProposedBlock currently rebuilds the whole note tree from individual notes, and so the per-batch trees are not used; OutputNoteBatch (= Vec<(usize, OutputNote)>) is the per-batch note list it keeps.

Proposed change:

• Build the BlockNoteTree by inserting each batch's BatchNoteTree as a subtree via BlockNoteTree::insert_batch_note_subtree(batch_idx, batch_note_tree), instead of rebuilding it note-by-note
• Remove block-level note erasure: stop erasing output notes across batches. The NoteTracker is still needed at the block level for duplicate detection, unauthenticated-note authentication, and the circular-dependency check, but it must no longer erase output notes (otherwise the per-batch BatchNoteTree roots would no longer match).

References

• PR feat: batch kernel skeleton #2904 (batch kernel skeleton)
• PR feat(batch kernel): wire up INPUT_NOTES_COMMITMENT & note erasure #2905 (batch kernel wiring)
• Discussion #3008