Integrate Garden Finance swapper for BTC ↔ wrapped BTC swaps

[swdiscordia]

Problem

ShapeShift Web does not currently support direct BTC ↔ wrapped-BTC swaps across the major BTC-pegged variants (wBTC, cbBTC, ibtc, BTCB, btc.b, ubtc, strkBTC). Users wanting to move between native Bitcoin and a wrapped variant must route through multiple swappers or external bridges.

Proposal

Integrate Garden Finance as a new swapper. Garden uses an HTLC + intent-solver model with a deposit-to-address flow (same shape as NearIntentsSwapper), and covers 24 BTC-related assets across 11 chains:

• UTXO sources: Bitcoin, Litecoin
• EVM sources: Ethereum, Base, BNB Chain, Arbitrum, Monad, HyperEVM, MegaETH
• Starknet source: WBTC + strkBTC (Garden's native Starknet BTC peg, launched May 2026)
• Destination-only (gasless flow not yet wired): Solana, Tron

Operational requirements

• Create partner account at https://portal.garden.finance/ and claim a garden-app-id API key.
• Behind a feature flag (VITE_FEATURE_GARDEN_SWAP) — disabled in production until governance sign-off given Garden's 2025-10 solver-layer exploit history (user funds unaffected per Garden + EY post-mortem; HTLC contracts audited by Trail of Bits Nov 2025).
• Affiliate fees claimed weekly via GET /apps/earnings, paid in base:cbbtc to DAO_TREASURY_BASE.

Scope

PR opens in tandem; this issue tracks the integration for governance / ops visibility.