implement btc/bch/zcash signing in rust

[0xh3rman]

Bitcoin Signer Test Matrix

Scope: core branch bitcoin-signer — migration of Bitcoin/BCH/Litecoin/Doge/Zcash signing from Trust Wallet Core to crates/gem_bitcoin::signer.

Goal: prove on real networks that signatures verify, fees are sensible, and change/dust handling never loses funds. Unit tests in gem_bitcoin cover preimage shape and byte-exact vectors; this matrix covers what only mainnet/testnet can confirm.

Transfer Matrix

ID	Coverage	Builder / branch	Example	Status
T1	Native segwit P2WPKH transfer with change	sign_bitcoin_like + UtxoPlanner::Exact	BTC bc1q → bc1q	Tested: BTC bc1qe7q… → bc1qw0e… (9f71a30b…) 2 in → 2 out (payment + change), 208 vB, fee 418 sat (~2 sat/vB)
T2	Max-send (sweep) leaving no change	UtxoPlanner::Max	BTC/LTC max-send	Tested: Doge (b99c4551…) 1-in, LTC (009a0abc…) 1-in, BTC (5072b15a…) 3-in. All 1 output, no change
T3	Sub-dust change absorbed into fee	try_build_plan no-change branch (spend.rs:144-152)	LTC send chosen so leftover < 546 sat
T4	Transfer with memo (OP_RETURN at output index 1)	op_return_output + spend_outputs	Covered indirectly by W1
T5	Legacy P2PKH transfer	sign_bitcoin_like legacy path	Doge D… → D…	Tested: Doge max-send (b99c4551…) D8g1MD… → DL8fYJ…, 191 vB, fee 0.0193 DOGE
T6	Bitcoin Cash P2PKH with FORKID sighash	bitcoin_cash::sign_plan (BIP143 + 0x41)	BCH bitcoincash:q… → bitcoincash:q…	Tested + confirmed at block 952,079: BCH max swap to SOL via NEAR Intents (7360dbab…) 192 B, fee 1,920 sat (~10 sat/B)
T7	Zcash t1 → t1 transparent transfer	zcash::sign_transparent ZIP-244 v5	Zcash t1… → t1…	Tested: t1RNsP3w… → t1dTLYue… + change (09cc92df…) v5, 240 B, fee 10,000 zat (ZIP-317 min, 2 actions). Mempool-accepted → sighash verifies at consensus level
T8	Zcash max-send	Zcash branch in UtxoPlanner::Max + ZIP-317 fee	Zcash sweep	Tested + block-confirmed: (e5d68d6b…) block 3,351,057, v5, 2 in → 1 out, no change, 355 B, fee 10,000 zat (ZIP-317 floor)

Swap Matrix

ID	Coverage	Builder / branch	Example	Status
W1	Swap via Transfer path (memo OP_RETURN), exact amount	SpendRequest::swap Transfer	BTC → ETH via Thorchain	Blocked on Thorchain resumption; unit test test_sign_swap_memo_op_return covers the path
W2	Swap via Transfer path, max amount (memo optional)	Transfer + use_max_amount	BCH → SOL via NEAR Intents max	Tested: BCH → SOL via NEAR Intents (7360dbab… confirmed at block 952,079 + intent 1JSnFERGiXnamsvNCPNQapoFpa1sU61S5N — NEAR indexer pending SOL credit
W3	Swap via Contract path (hex-decoded nulldata), exact amount	SpendRequest::swap Contract (request.rs:42-58)	BTC → SOL via Chainflip
W4	Swap via Contract path, max-intent (must still emit change as refund)	Contract + exact-amount with change	BTC → SOL via Chainflip max	FAILED then fixed: (5519a6b5…) confirmed on Bitcoin but rejected by Chainflip CFE — only 2 outputs (deposit + OP_RETURN), missing mandatory 3rd output that Chainflip uses as refund address. Fix: force is_max=false for Contract data type in request.rs; tests updated to require 3-output structure. Pending re-test

[gemini-code-assist]

Code Review

This pull request implements signing support for Bitcoin, Bitcoin Cash, Litecoin, Dogecoin, and Zcash, introducing a new BitcoinChainSigner and a transaction planner with fee estimation. The review feedback identifies critical performance improvements, such as precomputing sighash components to prevent O(N^2) complexity and optimizing the use of cryptographic contexts. Additionally, the reviewer recommends implementing dust threshold checks for transaction amounts and utilizing standard library abstractions for address parsing to enhance code reliability and maintainability.