perf(litebe): memoize per-note viewing-key decode on wallet open

Opening a wallet reads each shielded note from disk, and every note
record embeds a 169-byte extended full viewing key that was decoded
from scratch for every note. That decode is an elliptic-curve parse;
a wallet with many notes sharing a handful of keys paid for it once
per note, which dominated open time.

- Note reads now go through a small content-addressed cache keyed on
  the exact 169 raw key bytes. The first note with a given key
  decodes it; the rest reuse the result. Because the cache is keyed
  on the raw bytes and the decoder is deterministic, the cached key
  is identical to a fresh parse.
- The cache is thread-local and naturally bounded by the few distinct
  keys a wallet holds, so it adds no cross-thread state and cannot
  grow without bound.

The bytes on disk are read exactly as before - this only avoids
re-parsing identical bytes - so the loaded wallet is byte-identical,
verified against the differential harness. The wallet file format is
unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -1,5 +1,7 @@
 use std::io::{self, Read, Write};
 use std::sync::Arc;
+use std::cell::RefCell;
+use std::collections::HashMap;
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 use pairing::bls12_381::{Bls12};
@@ -26,6 +28,24 @@ use zcash_primitives::{
 use zcash_primitives::zip32::ExtendedSpendingKey;
 
 
+// C241: wallet open re-parsed the 169-byte ExtendedFullViewingKey once PER NOTE. Memoize the decode,
+// keyed by the exact 169 raw bytes (content-addressed, so the result is always identical to a fresh
+// parse). Thread-local and naturally bounded by the handful of distinct keys a wallet holds.
+thread_local! {
+    static EFVK_CACHE: RefCell<HashMap<[u8; 169], ExtendedFullViewingKey>> = RefCell::new(HashMap::new());
+}
+
+fn read_efvk_cached<R: Read>(r: &mut R) -> io::Result<ExtendedFullViewingKey> {
+    let mut raw = [0u8; 169];
+    r.read_exact(&mut raw)?;
+    if let Some(k) = EFVK_CACHE.with(|c| c.borrow().get(&raw).cloned()) {
+        return Ok(k);
+    }
+    let k = ExtendedFullViewingKey::read(&raw[..])?; // identical parser on identical bytes
+    EFVK_CACHE.with(|c| c.borrow_mut().insert(raw, k.clone()));
+    Ok(k)
+}
+
 pub struct BlockData {
     pub height: i32,
     pub hash: BlockHash,
@@ -147,7 +167,7 @@ impl SaplingNoteData {
 
         let account = reader.read_u64::<LittleEndian>()? as usize;
 
-        let extfvk = ExtendedFullViewingKey::read(&mut reader)?;
+        let extfvk = read_efvk_cached(&mut reader)?;
 
         let mut diversifier_bytes = [0u8; 11];
         reader.read_exact(&mut diversifier_bytes)?;