PASS1-527: Add BIP375 Silent Payment sending support

[220nightmore-spec]

Summary

Implements sender-side Silent Payments support for Passport firmware using BIP352, BIP374, and the standardized BIP375 PSBTv2 protocol.

• decodes and encodes mainnet/testnet Silent Payment addresses;
• parses, reconstructs, and serializes BIP370 PSBTv2 transactions;
• parses and serializes BIP375 global, per-input, and per-output fields;
• derives and verifies Silent Payment output scripts before transaction review;
• supports all-Passport inputs with compact global ECDH shares;
• supports collaborative transactions by validating external per-input DLEQ proofs and adding Passport-owned per-input shares;
• exports an unsigned share-only intermediate PSBT when another signer has not contributed its required share yet;
• displays the original sp... / tsp... destination during confirmation; and
• preserves the existing PSBTv0 signing path.

Closes #527.

Protocol and safety

The implementation follows BIP375 rather than defining a proprietary host/device field:

• every eligible input is bound to its P2PKH/P2WPKH/P2SH-P2WPKH or Taproot prevout public key;
• the BIP341 NUMS Taproot internal-key exception is excluded from shared-secret derivation;
• supplied global and per-input shares are accepted only after BIP374 DLEQ verification;
• output scripts are recomputed and compared before signing;
• inputs and outputs are locked after missing scripts are computed;
• ECDSA, Taproot key-path, and Taproot script-path signatures are restricted to SIGHASH_ALL;
• incomplete collaborative PSBTs cannot reach the signing task; and
• derived private keys are blanked on success and failure paths.

Validation

• 31 focused firmware-module tests;
• all 14 finalizable vectors from macgyver13/bip375-test-generator v1.1.1, covering labeled outputs, global/per-input shares, collaborative inputs, and excluded P2SH/NUMS inputs;
• exact output-script and PSBT round-trip match against the official BIP375 single-signer vector;
• all 11 official BIP374 proof-generation vectors;
• all 15 official BIP374 proof-verification vectors;
• all 28 BIP352 v1.1.1 sending cases for the sender core;
• py_compile, repository pycodestyle, and git diff --check.

All tests use deterministic public vectors. No funded transaction, real seed, wallet credential, or user-private material is involved.

Known limitation

PSBT_OUT_SP_V0_LABEL is preserved and labeled outputs are derived and verified correctly, but Passport does not yet prove that a labeled Silent Payment output belongs to its own scan/spend derivation. Such outputs are therefore shown conservatively as payments instead of being hidden as trusted change.

[220nightmore-spec]

Progress update: commit b2c8672b completes the collaborative BIP375 round-trip and the PR is now ready for review.

The branch now covers PSBTv2 parsing/serialization, BIP375 global and per-input proof handling, labeled output derivation, output-script verification, SIGHASH_ALL enforcement, original Silent Payment address confirmation, collaborative input verification, and share-only intermediate PSBT export when another signer has not supplied its share yet. Incomplete collaborative PSBTs are explicitly prevented from reaching the signing task.

Validation completed:

• 31 focused firmware-module tests;
• all 14 finalizable vectors from the current BIP375 generator v1.1.1;
• exact output-script and PSBT round-trip match against the official BIP375 single-signer vector;
• all 11 official BIP374 proof-generation vectors;
• all 15 official BIP374 proof-verification vectors;
• all 28 BIP352 v1.1.1 sender cases; and
• py_compile, repository pycodestyle, and git diff --check.

The remaining known limitation is conservative UI classification of labeled Silent Payment change: scripts are derived and verified, but ownership is not proven from Passport scan/spend derivations, so these outputs are shown as payments instead of hidden as trusted change.