refactor(swap): replace atomic two-leg intercept with HODL-invoice flow

[Arshia-r-m]

The previous swap mechanism required three coordinated calls (maker_init → taker whitelist → maker_execute) and a custom HTLCIntercepted path keyed on an IS_SWAP_SCID flag to fire both legs atomically.

Swaps now use a HODL BOLT11 invoice plus a forward keysend:

• maker_init takes a taker_pubkey, generates its own preimage, builds a HODL invoice for qty_from, and stores the swap (taker_pubkey lives in SwapInfo at TLV 5, and the swapstring is extended to 7 fields).
• The taker pays the invoice with plain /sendpayment — no whitelist.
• On PaymentClaimable for a maker-swap HODL, the maker validates the incoming amount and fires a single-leg spontaneous payment (carrying the RGB asset when qty_to is RGB) to taker_pubkey, encoding the HODL hash in PaymentId so the follow-up events can correlate.
• PaymentSent on the forward leg triggers claim_funds on the HODL; PaymentClaimed finalises the swap and updates the RGB channel balance. PaymentFailed (or an immediate send failure) calls fail_htlc_backwards to refund the taker and marks the swap Failed.

Deletes maker_execute, the /taker whitelist requirement (the endpoint is kept for optional taker-side bookkeeping), the HTLCIntercepted swap branch, IS_SWAP_SCID, and the swap_roundtrip_fail_whitelist test. Also drops a handful of now-dead helpers (get_route, Router/Scorer type aliases, DEFAULT_FINAL_CLTV_EXPIRY_DELTA, the UnlockedAppState.router field) that were only reachable from the removed paths.

All swap tests are rewritten around the new helpers (taker_pay_invoice replaces taker + maker_execute) and channel- balance assertions are updated to reflect that each leg is now a separate LN payment with a single forwarding fee at the middle hop (and sub-dust receiver balances for tiny-msat forward legs).

Trade-off: the atomic two-HTLC property is gone. If the maker crashes after the HODL is claimable but before claim_funds runs, manual recovery via the existing claim_hodl_invoice endpoint is required — the standard HODL-invoice tradeoff.

Verified: cargo build --tests is warning-free; all 17 swap tests pass (1003s wall time).