perf(lite-backend): share unchanged witness states between blocks

Each scanned block deep-copied the newest witness of every note with
history to start that block's entry, although a block without shielded
outputs changes no witness, and spent or pending notes never change
theirs. A note's 100-entry list was 100 separate copies, mostly
identical.

The list now holds Arc-shared witnesses. The per-block duplicate is an
Arc::clone, and Arc::make_mut copies the newest entry only for unspent,
non-pending notes and only in blocks with at least one shielded output.
Every entry is still written in full, so the file is unchanged, and a
spend takes its own copy of the witness. A loaded wallet is not
de-duplicated, so the saving builds up over the next 100 blocks.

No output change: the saved bytes are identical; notes use less memory.
With 1000 live notes x 100 witnesses (a 70 MB file), RssAnon after 100
blocks without outputs is 18 MiB instead of 189 MiB and such a block
scans in 1.0 ms instead of 2.1-2.7 ms; when every block has an output,
the live bytes stay about the same.

The witness tests, the 1000-seed oracle and a differential run against
the unmodified backend (every witness entry and the whole saved file
after every operation; only the cleared lists and the chosen spend notes
may differ) all pass, and the measured wallet saves to the same SHA-256
as with the old build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

third_party/silentdragonxlite/silentdragonxlite-cli/lib/src/lightwallet.rs

@@ -2304,10 +2304,10 @@ pub fn scan_full_mempool_tx(&self, tx: &Transaction, height: i32, _datetime: u64
             // Prepare the note witnesses for updating
             for tx in txs.values_mut() {
                 for nd in tx.notes.iter_mut() {
-                    // Duplicate the most recent witness (nothing to do for notes with no history)
+                    // Duplicate the most recent witness (shares the allocation; deep-cloned only when appended to)
                     if let Some(witness) = nd.witnesses.last() {
-                        let clone = witness.clone();
-                        nd.witnesses.push(clone);
+                        let shared = Arc::clone(witness);
+                        nd.witnesses.push(shared);
                         // Trim the oldest witnesses in place (no fresh Vec per note per block)
                         let len = nd.witnesses.len();
                         if len > MAX_REORG {
@@ -2322,12 +2322,18 @@ pub fn scan_full_mempool_tx(&self, tx: &Transaction, height: i32, _datetime: u64
                 let extfvks: Vec<ExtendedFullViewingKey> = self.zkeys.read().unwrap().iter().map(|zk| zk.extfvk.clone()).collect();
 
                 // Create a single mutable slice of all the newly-added witnesses.
-                let mut witness_refs: Vec<_> = txs
-                    .values_mut()
-                    .map(|tx| tx.notes.iter_mut().filter_map(
-                        |nd| if nd.spent.is_none() && nd.unconfirmed_spent.is_none() { nd.witnesses.last_mut() } else { None }))
-                    .flatten()
-                    .collect();
+                // A block without shielded outputs appends nothing to any witness: don't un-share (deep-clone) them.
+                let block_has_outputs = block.vtx.iter().any(|t| !t.outputs.is_empty());
+                let mut witness_refs: Vec<&mut IncrementalWitness<Node>> = if block_has_outputs {
+                    txs
+                        .values_mut()
+                        .map(|tx| tx.notes.iter_mut().filter_map(
+                            |nd| if nd.spent.is_none() && nd.unconfirmed_spent.is_none() { nd.witnesses.last_mut().map(|w| Arc::make_mut(w)) } else { None }))
+                        .flatten()
+                        .collect()
+                } else {
+                    Vec::new()
+                };
 
                 self.scan_block_internal(
                     block.clone(),

third_party/silentdragonxlite/silentdragonxlite-cli/lib/src/lightwallet/data.rs

@@ -1,4 +1,5 @@
 use std::io::{self, Read, Write};
+use std::sync::Arc;
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 use pairing::bls12_381::{Bls12};
@@ -66,7 +67,8 @@ pub struct SaplingNoteData {
     pub(super) extfvk: ExtendedFullViewingKey, // Technically, this should be recoverable from the account number, but we're going to refactor this in the future, so I'll write it again here.
     pub diversifier: Diversifier,
     pub note: Note<Bls12>,
-    pub(super) witnesses: Vec<IncrementalWitness<Node>>,
+    // One entry per block (<= MAX_REORG), oldest first. Entries of blocks that appended nothing share one allocation.
+    pub(super) witnesses: Vec<Arc<IncrementalWitness<Node>>>,
     pub(super) nullifier: [u8; 32],
     pub spent: Option<TxId>,             // If this note was confirmed spent
     pub unconfirmed_spent: Option<TxId>, // If this note was spent in a send, but has not yet been confirmed.
@@ -130,7 +132,7 @@ impl SaplingNoteData {
             extfvk: extfvk.clone(),
             diversifier: output.to.diversifier,
             note: output.note,
-            witnesses: vec![witness],
+            witnesses: vec![Arc::new(witness)],
             nullifier: nf,
             spent: None,
             unconfirmed_spent: None,
@@ -162,7 +164,7 @@ impl SaplingNoteData {
             None     => Err(io::Error::new(io::ErrorKind::InvalidInput, "Couldn't create the note for the address"))
         }?;
 
-        let witnesses = Vector::read(&mut reader, |r| IncrementalWitness::<Node>::read(r))?;
+        let witnesses = Vector::read(&mut reader, |r| IncrementalWitness::<Node>::read(r).map(Arc::new))?;
 
         let mut nullifier = [0u8; 32];
         reader.read_exact(&mut nullifier)?;
@@ -553,7 +555,7 @@ impl SpendableNote {
                 nullifier: nd.nullifier,
                 diversifier: nd.diversifier,
                 note: nd.note.clone(),
-                witness: w.clone(),
+                witness: (**w).clone(),
                 extsk: extsk.clone().unwrap(),
             })
         } else {