TST: add unit tests for the Rust BIP-352 silent-payment scanner

[NAME-ASHWANIYADAV]

Proposing a feature?

rust_jsi_bridge/src/lib.rs is the cryptographic core of the wallet — it does all BIP-352 scanning (ECDH, tagged hashing, output-key derivation) and is consumed from TypeScript over the C FFI. It currently has zero tests, and no CI workflow runs cargo test.

This is the highest-consequence untested code in the repo. A silent regression in the scanner means either missed payments (funds look lost) or a matched UTXO whose returned tweakHex cannot actually derive a spending key.

What I'd like to add

An inline #[cfg(test)] mod tests in lib.rs covering:

• compute_shared_secret_hash — the BIP0352/SharedSecret tagged hash and its
  big-endian output-index encoding
• ecdh_shared_secret — sender/receiver ECDH duality
• derive_expected_pubkey — P_k = B_spend + t_k*G
• parse_scan_tweak — all error paths
• build_output_map, scan_outputs, scan_transaction
• process_transactions_parallel — counters and rayon output ordering
• The serde wire contract with modules/RustJsiBridge.ts (camelCase keys)
• The FFI boundary — null pointers, error wrapping, the embedded-NUL fallback

Plus a small workflow so the tests actually gate PRs.

On not writing circular tests

The obvious approach — build a fixture with derive_expected_pubkey, then assert scan_transaction finds it — is worthless: it passes even if the crypto is completely wrong. So every expected value would be computed from the sender's side of BIP-352 using different secp256k1 APIs than the scanner uses (mul_tweak/combine against shared_secret_point/add_exp_tweak), and the tagged hash rebuilt by hand from raw sha256, bypassing the sha256t_hash_newtype! macro.

Notes

• No new dependencies — everything is doable with the existing secp256k1, bitcoin_hashes, hex, and serde_json.
• No production changes. Cargo.toml already has crate-type = [..., "lib"] ("Rust library for tests and examples"), so cargo test works as-is.
• Tests go inline rather than in tests/ because nearly every function under test is private.