feat(zcash-wasm): PCZT producer + extractor + redactor + UR fountain decoder

[hitchho]

Summary

• Adds the cold-signer signing pipeline that pairs zafu with any UR/PCZT-compatible hardware (zigner, Keystone, zashi).
• Closes the display-vs-sighash gap: the cold device now recomputes the sighash from PCZT contents itself, binding the displayed plan to the signed bytes by construction.
• Bumps the librustzcash dep slice to git rev 5333c01b across orchard / zcash_primitives / zcash_keys / zcash_protocol / zcash_transparent / sapling-crypto so versions align with the pczt crate at the same rev (which zigner and zashi-android also consume).

What lands

API	Purpose
build_unsigned_pczt	Builder → Creator → Prover (Halo 2) → IoFinalizer → redactor → serialize. Returns canonical pczt::Pczt::serialize() bytes.
extract_signed_tx_from_pczt(_bytes)	TransactionExtractor over a fully-signed PCZT → broadcast-ready v5 hex.
redact_pczt_for_signer	Strips fields the cold device doesn't need (witness, zip32_derivation, dummy_sk, proprietary). Preserves everything the signer reconstructs the sighash from.
ur_decode_frames	BC-UR multipart fountain decoder for the receive path.

Tests

• pczt_redactor_smoke — source-grep guard that build_unsigned_pczt invokes the redactor in the right pipeline order.
• pczt_redactor_property — 4 behavioral tests on a synthetic transparent → orchard PCZT: round-trip stability, size shrinks under redaction, Signer::new still accepts redacted PCZT (kept fields intact), shielded sighash is invariant under redaction (the security property the migration delivers).
• extract_signed_tx — full prove + sign + extract end-to-end with structural validation of the v5 tx bytes (header words, version_group_id, vin/vout/orchard shape, re-parses via Transaction::read).
• zashi_interop_snapshot — scaffold for #[ignore]'d external-fixture test; drop a real zashi/keystone-emitted ur:zcash-pczt byte stream into tests/fixtures/ to activate.

Test plan

• cargo test --release --test pczt_redactor_smoke --test pczt_redactor_property --test extract_signed_tx --test zashi_interop_snapshot — 6 passing, 1 ignored (pending external fixture).
• wasm-pack build --release --target web --no-default-features — clean.
• Parallel rayon build via RUSTFLAGS='-C target-feature=+atomics,+bulk-memory,+mutable-globals,+simd128' cargo +nightly build -Z build-std=panic_abort,std --features parallel — clean.
• On-device hardware test against real zigner — pending companion zafu PR.
• Real Keystone interop test — pending hardware in loop + fixture drop.

🤖 Generated with Claude Code

[hitchho]

Review round (8-persona audit) + corrections

An 8-reviewer audit was run. Walking back overstated claims and recording what changed.

Claims corrected:

• "binding the displayed plan to the signed bytes by construction" → the sighash↔redaction binding is by construction (hdevalence confirmed: the redactor provably never touches a pczt_to_tx_data input). But producer-side merkle/anchor handling was not by construction. Drop the unqualified absolute.
• "the security property the migration delivers" parenthetical on the redactor test → scope it to redaction-invariance only; it is not the whole security property.
• The zashi_interop_snapshot test is #[ignore]d pending an external fixture — no executed interop coverage exists yet. Stated honestly here.

Fixed this round (commit 2038eb6):

• validate_ufvk — authoritative UnifiedFullViewingKey::decode export; the single UFVK decoder, called at zafu's persistence boundary (no divergent hand-rolled validator).
• merkle filter_map → positional decode with precise sibling i/j not canonical: <hex> errors at both sites. Not a silent-corruption hole (orchard has_matching_anchor catches a wrong path) but it was undiagnosable on hardware.

Disposition: keep as Draft. Merge after a behavioral merkle-rejection test lands (now testable post-fix) and the companion zafu anchor-depth fix is hardware-validated. Reciprocal note: rotkonetworks/zafu#11 vendors wasm built from this branch — see its BUILD_PROVENANCE.md pinning the exact rev.