Cross-chain: Orchestra receive

[dangeross]

Adds Orchestra-backed cross-chain receive: external chain → Spark (BTC sats or USDB). Companion to the existing cross-chain send.

What's new

• New ReceivePaymentMethod::CrossChain { route, amount, destination, fee_mode, max_slippage_bps, target_overpay_bps } variant.
• amount is denominated in USDB base units (6-decimal) regardless of destination or fee mode ($1 = 1_000_000), so callers never touch destination decimals. FeesExcluded (default) treats it as the receiver's net target and the SDK sizes the sender's deposit. FeesIncluded treats it as the deposit and the receiver lands amount − fees.
• destination auto-picks the wallet's active stable-balance token if the route supports it, otherwise Bitcoin. Explicit values must appear in route.accepted_assets.
• ReceivePaymentResponse.cross_chain_info carries the deposit amount, expected received amount, destination denomination, provider fee (service_fee_amount + service_fee_asset, symmetric with the send-side field), and quote expiry.
• New CrossChainService::prepare_receive trait method. Orchestra implements it; Boltz returns InvalidInput.

Breaking changes

Two enums and two CrossChainRoutePair fields are renamed so the names read naturally from both send and receive:

• SourceAsset → SparkAsset (enum)
• SourceChain → DeliveryMethod (enum)
• CrossChainRoutePair.supported_sources → accepted_assets
• CrossChainRoutePair.supported_source_chains → delivery_methods

Renames propagate through UniFFI / wasm-bindgen / FRB, so downstream integrators referencing the old names will need to update.

Notes

• USDB-denominated amount is a deliberate choice (integrator ergonomics over per-token decimals) but diverges from prepare_payment_link, which uses destination-native units. Worth reviewer input.