PIR spendability integration

[p0mvn]

Use Private Information Retrieval to detect spent notes and fetch witnesses before sync completes, giving users early visibility into pending spends. Adds the full data flow (SDK synchronizer interface, Root reducer orchestration, shared state), a PIR setup screen in Advanced Settings with a persisted user toggle, and pending-spend UI treatment in transaction rows.

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.

Design Assumptions

• Spendability check is triggered when the user opens the app for all notes in the wallet at the time
• Keystone / PCZT path is kept out of the initial scope
• The only new UI components are configuration in Advanced Settings (screenshots attached)
• The main user-facing side-effect is immediate spendability (sub 10 seconds for 2 notes) when starting up
• When spending, all notes must reference the same anchor height. The anchor height is refreshed right before confirmation and chosen for spending.
• There is an automated retry mechanism if the broadcasting fails due to invalid anchor height

Design Document

https://github.com/valargroup/spendability-pir/blob/main/docs/pir_wallet_integration.md

Video Demo

https://screen.studio/share/tm6q8yUz

Screenshots

Related PRs

• PIR spendability integration #32
• Draft: Add PIR-based spendability checking and witness fetching zcash-swift-wallet-sdk#15
• Draft: Add PIR spendability support to zcash_client_sqlite librustzcash#13

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[p0mvn]

Note: Some of these changes are intentionally left for convenience when continuing to pick this up