Add PIR spendability support to zcash_client_sqlite

[p0mvn]

Branched from maint/zcash_client_sqlite-0.19.x (post zcash_client_sqlite-0.19.5).

https://screen.studio/share/tm6q8yUz

Motivation

During normal wallet sync, Orchard notes are not spendable until the
shard-tree scanner has processed enough blocks to construct a Merkle
authentication path for each note. For wallets that are catching up after
being offline, this can mean a significant delay before funds become
available.

Nullifier PIR (Private Information Retrieval) sidesteps this by querying
an external server for two pieces of information:

1. Nullifier inclusion — has a note been spent on-chain? This lets the
   wallet mark notes as spent before the scanner confirms it.
2. Merkle authentication paths — the server provides the witness data
   needed to spend a note, so the wallet does not need to wait for its
   local shard to be fully scanned.

Together, these allow the wallet to display accurate spendable balances
and build transactions within seconds of startup, rather than waiting for
a full scan.

Changes

zcash_client_backend

• WalletCommitmentTrees::get_pir_orchard_merkle_path — new trait
  method (with default no-op returning Ok(None)) for retrieving a
  PIR-provided Orchard Merkle path by commitment tree position.
• PirOrchardWitness type alias for the witness tuple.
• spendability-pir feature flag gating the above.
• create_proposed_transactions gains a use_pir_witnesses parameter
  (feature-gated) that switches the Orchard witness source from the
  local ShardTree to stored PIR witnesses.

zcash_client_sqlite

• New wallet::spendability_pir module with:
  • Nullifier gate queries (get_unspent_orchard_notes_for_pir)
  • Witness storage, retrieval, and validation (insert_pir_witness,
    get_pir_witness, validate_orchard_witness, etc.)
  • Helper types: UnspentOrchardNote, NoteNeedingWitness,
    PirWitnessRow, PirWitnessedNote, PirWitnessValidation
• spendability_pir_tables migration creating the pir_witness_data
  table. The migration is unconditional (not feature-gated) to keep the
  migration DAG identical across all builds; the table is empty and
  unused when the feature is off.
• get_wallet_summary treats Orchard notes with PIR witnesses as
  spendable even when their shard is not fully scanned.
• Note selection (select_spendable_notes) accepts Orchard notes with
  PIR witnesses, bypassing the shard-scanned gate.
• truncate_to_height unconditionally clears pir_witness_data to
  invalidate authentication paths after reorgs.
• WalletCommitmentTrees::get_pir_orchard_merkle_path implemented for
  WalletDb, backed by the pir_witness_data table.
• CHANGELOG updated under [Unreleased].