Move bitcoin signer to rust

[0xh3rman]

Transfer Matrix

ID	Coverage	Builder / branch	Example	Status
T1	Native segwit P2WPKH transfer with change	sign_bitcoin_like + UtxoPlanner::Exact	BTC bc1q → bc1q	Tested (9f71a30b…)
T2	Max-send (sweep) leaving no change	UtxoPlanner::Max	BTC/LTC max-send	Tested — BTC/LTC/Doge (5072b15a…)
T4	Transfer with memo (OP_RETURN at output index 1)	op_return_output + spend_outputs	Covered indirectly by W1	Via W1
T5	Legacy P2PKH transfer	sign_bitcoin_like legacy path	Doge D… → D…	Tested (b99c4551…)
T6	Bitcoin Cash P2PKH with FORKID sighash	bitcoin_cash::sign_plan (BIP143 + 0x41)	BCH bitcoincash:q… → bitcoincash:q…	Tested (7360dbab…)
T7	Zcash t1 → t1 transparent transfer	zcash::sign_transparent ZIP-244 v5	Zcash t1… → t1…	Tested (09cc92df…)
T8	Zcash max-send	Zcash branch in UtxoPlanner::Max + ZIP-317 fee	Zcash sweep	Tested (e5d68d6b…)

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	Pending (Thorchain paused)
W2	Swap via Transfer path, max amount (memo optional)	Transfer + use_max_amount	BCH → SOL via NEAR Intents max	Tested (7360dbab…, intent)
W3	Swap via Contract path (hex-decoded nulldata), exact amount	SpendRequest::swap Contract (request.rs:42-58)	BTC → SOL via Chainflip	Tested + credited (scan, 9931d6dd…)
W4	Swap via Contract path, max-intent (must still emit change as refund)	Contract + exact-amount with change	BTC → SOL via Chainflip max	Fixed + tested (scan, 759ef7a6…)