fix: preserve raw nTxType bytes on pre-DIP-0002 transactions

[QuantumExplorer]

Summary

Follow-up to #675. That PR made version == 0 transactions decode as Classic so they wouldn't be rejected on UnknownSpecialTransactionType, but it dropped the original on-wire nTxType u16. After decode, consensus_encode() and txid() re-emitted self.tx_type() as 0, so the malformed mainnet transaction no longer round-tripped and its txid changed after decode.

This PR introduces a new variant pair that preserves the raw u16:

• TransactionType::ClassicalWithNonStandardVersionTypeBytes(u16)
• TransactionPayload::ClassicalWithNonStandardVersionTypeBytesPayloadType(u16)

The pseudo-payload variant carries the raw bytes through the existing Option<TransactionPayload> slot on Transaction, so no new struct field is added. tx_type() derives the TransactionType variant from it via the existing from_optional_payload path.

Encoder / txid / sighash

• (self.tx_type() as u16) is replaced with self.tx_type().to_u16() (had to drop #[repr(u16)] since the enum now has a tuple variant).
• The encoder, txid(), and the sighash path skip the payload-section block when the payload is the new variant — pre-DIP-0002 transactions have no payload section on the wire (not even a length prefix), so emitting one would change the bytes.

Decoder

For version == 0:

• type bytes == 0 → Classic (existing behavior preserved)
• type bytes != 0 → ClassicalWithNonStandardVersionTypeBytes(raw) (new path)

Wallet routing

TransactionRouter::classify_transaction treats the new variant as logically Classic — falls through to the standard / coinbase / coinjoin classification.

Test plan

• New unit test test_pre_dip2_classical_with_non_standard_version_type_bytes_roundtrip:
  • decodes a tx with version=0, nTxType=0x002A and asserts the variant is preserved
  • asserts byte-exact serialize round-trip
  • asserts txid() == Txid::hash(tx_bytes)
  • asserts the txid differs from the same tx with type bytes zeroed (proves we don't collapse different on-chain txs to one id)
• Existing test_transaction_version still passes (version=0, type=0 still maps to plain Classic)
• cargo test --workspace --lib — 532 dashcore + 461 key-wallet + others all pass
• cargo clippy --workspace --all-features --all-targets -- -D warnings clean
• cargo fmt --check clean

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 95377d5 and 68f5593.

📒 Files selected for processing (6)

• dash/src/blockdata/transaction/mod.rs
• dash/src/blockdata/transaction/special_transaction/asset_unlock/unqualified_asset_unlock.rs
• dash/src/blockdata/transaction/special_transaction/mod.rs
• dash/src/crypto/sighash.rs
• key-wallet/src/psbt/map/global.rs
• key-wallet/src/transaction_checking/transaction_router/mod.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/recursing-leakey-fd3cad

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[codecov]

Codecov Report

❌ Patch coverage is 78.26087% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.86%. Comparing base (7dc1225) to head (68f5593).
⚠️ Report is 1 commits behind head on v0.42-dev.

Files with missing lines	Patch %	Lines
...c/blockdata/transaction/special_transaction/mod.rs	51.42%	17 Missing ⚠️
...src/transaction_checking/transaction_router/mod.rs	78.57%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           v0.42-dev     #726      +/-   ##
=============================================
+ Coverage      70.84%   70.86%   +0.02%     
=============================================
  Files            319      319              
  Lines          68184    68243      +59     
=============================================
+ Hits           48305    48361      +56     
- Misses         19879    19882       +3     

Flag	Coverage Δ
core	75.82% <77.33%> (+0.01%)	⬆️
ffi	45.49% <ø> (ø)
rpc	20.00% <ø> (ø)
spv	87.24% <ø> (+0.05%)	⬆️
wallet	69.61% <82.35%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
dash/src/blockdata/transaction/mod.rs	86.76% <100.00%> (+0.58%)	⬆️
...ansaction/asset_unlock/unqualified_asset_unlock.rs	70.19% <100.00%> (ø)
dash/src/crypto/sighash.rs	65.61% <100.00%> (ø)
key-wallet/src/psbt/map/global.rs	51.26% <100.00%> (+0.62%)	⬆️
...src/transaction_checking/transaction_router/mod.rs	88.80% <78.57%> (-0.46%)	⬇️
...c/blockdata/transaction/special_transaction/mod.rs	38.16% <51.42%> (+2.64%)	⬆️

... and 3 files with indirect coverage changes

[QuantumExplorer]

Self Reviewed