test(lite-backend): add a seeded chain oracle and offline spend verifier

No existing test checks that a send's anchor, nullifiers and proofs are
valid, or that note witnesses stay right through reorgs and reloads, and
that is what the following witness and note-selection changes touch.

oracle_tests.rs drives a LightWallet through its public entry points
over seeded random fake chains that start at real mainnet checkpoints:
receives to HD, diversified, imported, view-only and foreign keys,
zero-value and dust notes, outputs with an unparseable epk, compact
spends with change, real sends that are mined later or never (mempool
expiry), reorgs of 1-100 blocks in one call or block by block (re-mining
spends on the new branch), and save/reload.

An independent reference keeps its own commitment tree, every block's
root and every note's true witness at each height. After each step it
checks the tree and block window, every note's fields and spent state,
witness-list lengths, every retained witness of a live note byte for
byte (sampled ones also by root, position and auth path), the balances
and the mempool. Sends are captured in the broadcast closure, never
sent, and verified offline: the anchor must be the reference root at tip
minus the anchor offset, every nullifier must belong to an eligible
note, and the Sapling verifier must accept every spend and output proof
and the binding signature (the first send of a run also checks that it
rejects a wrong anchor and a wrong nullifier).

The default run (20 seeds x 300 blocks) is part of the normal suite; a
1000-seed x 400-block run and a 20-seed x 5000-block soak are #[ignore].
Every run prints its seed, and a single seed can be re-run.

Two pre-existing bugs are documented as #[ignore] reproducers and kept
out of the green runs:
- a reorg while a send is pending panics the backend (invalidate_block
  unwraps `spent` on a note that only has `unconfirmed_spent`);
- a pending send's notes stop receiving witness updates, so after a
  restart (the send never mined) or a reorg that undoes the block that
  mined it, they are spendable again with stale witnesses, and later
  sends from them fail until a rescan.

On the unchanged production code the default and full runs pass (1000
seeds: 12,194 reorgs up to 100 deep, 5,893 reloads, 993 proving sends
verified offline at anchor offsets 0 and 4) and the reproducers fail as
described.

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

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

@@ -2598,3 +2598,6 @@ pub fn scan_full_mempool_tx(&self, tx: &Transaction, height: i32, _datetime: u64
 
 #[cfg(test)]
 pub mod tests;
+
+#[cfg(test)]
+mod oracle_tests;