BIP375: Add test vectors + validator

This PR provides bip-0375/bip375_test_vectors.json and a reference bip-0375/validator/validate_psbt.py for validating Sending Silent Payments with PSBTs. PSBTs are validated against v2 requirements with BIP-375 rules (building on BIP-352 silent payment derivation and BIP-374 DLEQ proofs).

Changes since last force-push:

• Add vendored dependencies in bip-0375/deps:
  • secp256k1lab (ECC operations)
  • bitcoin_test (PSBT parsing adapted from Bitcoin Core test framework)
• Implement 4-stage sequential PSBT validation: structure, ECDH coverage + DLEQ correctness, input eligibility, and output script verification
• Expanded test vector coverage:
  • mixed + ineligible inputs
  • output script validation with same scan key / spend key, same scan / different spend key, different scan key
  • introduce P2TR inputs
• Add test vector version field (starting at 1.1)
• Update BIP-0375 Test Vectors section

Open Questions:

• BIP-375 allows PSBT_OUT_SCRIPT field to be optional if PSBT_OUT_SP_V0_INFO field is present for an output, should PSBT_OUT_SCRIPT field with empty value_data be considered valid? - I prefer invalid
• Should BIP text be clarified for Computing the Output Scripts as follows? - added 'from eligible inputs'

Compute the PSBT_OUT_SCRIPT using the procedure in BIP352 but substituting a·Bscan with the PSBT_GLOBAL_SP_ECDH_SHARE for that scan key if available, or the sum of all PSBT_IN_SP_ECDH_SHAREs from eligible inputs for that scan key.

Feedback welcome @andrewtoth @achow101 @theStack

<details> <h2>Test Runner Output</h2>

Description: BIP-375 Test Vectors Version: 1.1

Invalid PSBTs: 23 psbt structure: missing PSBT_OUT_SP_V0_INFO field when PSBT_OUT_SP_V0_LABEL set psbt structure: incorrect byte length for PSBT_OUT_SP_V0_INFO field psbt structure: incorrect byte length for PSBT_IN_SP_ECDH_SHARE field psbt structure: incorrect byte length for PSBT_IN_SP_DLEQ field psbt structure: PSBT_GLOBAL_TX_MODIFIABLE field is non-zero when PSBT_OUT_SCRIPT set for sp output psbt structure: missing PSBT_OUT_SCRIPT field when sending to non-sp output psbt structure: empty PSBT_OUT_SCRIPT field when sending to non-sp output ecdh coverage: only one ineligible P2MS input when PSBT_OUT_SCRIPT set for sp output ecdh coverage: missing PSBT_IN_SP_ECDH_SHARE field for input 0 when PSBT_OUT_SCRIPT set for sp output ecdh coverage: missing PSBT_IN_SP_DLEQ field for input when PSBT_IN_SP_ECDH_SHARE set ecdh coverage: missing PSBT_GLOBAL_SP_DLEQ field when PSBT_GLOBAL_SP_ECDH_SHARE set ecdh coverage: invalid proof in PSBT_IN_SP_DLEQ field ecdh coverage: invalid proof in PSBT_GLOBAL_SP_DLEQ field ecdh coverage: missing PSBT_IN_BIP32_DERIVATION field for input when PSBT_IN_SP_DLEQ set ecdh coverage: output 1 missing ECDH share for scan key with one input / three sp outputs (different scan keys) ecdh coverage: input 1 missing ECDH share for output 1 with two inputs / two sp outputs (different scan keys) ecdh coverage: input 1 missing ECDH share for scan key with two inputs / one sp output input eligibility: segwit version greater than 1 in transaction inputs with sp output input eligibility: non-SIGHASH_ALL signature on input with sp output output scripts: P2TR input with NUMS internal key cannot derive sp output output scripts: PSBT_OUT_SCRIPT does not match derived sp output output scripts: two sp outputs (same scan / different spend keys) not sorted lexicographically by spend key output scripts: k values assigned to wrong output indices with three sp outputs (same scan / spend keys)

Valid PSBTs: 18 can finalize: one input single-signer can finalize: two inputs single-signer using global ECDH share can finalize: two inputs single-signer using per-input ECDH shares can finalize: two inputs / two sp outputs with mixed global and per-input ECDH shares can finalize: one input / one sp output with both global and per-input ECDH shares can finalize: three sp outputs (different scan keys) with multiple global ECDH shares can finalize: one P2WPKH input / two mixed outputs - labeled sp output and BIP 32 change can finalize: one input / two sp outputs - output 0 has no label / output 1 uses label=0 convention for sp change can finalize: two sp outputs - output 0 uses label=3 / output 1 uses label=1 can finalize: two mixed input types - only eligible inputs contribute ECDH shares (P2SH excluded) can finalize: two mixed input types - only eligible inputs contribute ECDH shares (NUMS internal key excluded) can finalize: three sp outputs (same scan key) - each output has distinct k value can finalize: three sp outputs (same scan key) / two regular outputs - k values assigned independently of output index in progress: two P2TR inputs, neither is signed in progress: one P2TR input / one sp output with no ECDH shares when PSBT_OUT_SCRIPT field is not set in progress: two inputs / one sp output, input 1 missing ECDH share when PSBT_OUT_SCRIPT field is not set in progress: one input / two sp outputs, input 0 missing ECDH share for output 0 when PSBT_OUT_SCRIPT field is not set in progress: large PSBT with nine mixed inputs / six outputs - some inputs signed

Summary: 41 passed, 0 failed

</details>

Test vector generator is available in an external repo