feat(explorer): cross-chain token swap saga (/cross-swap)

[bachal-mb]

feat(explorer): cross-chain token swap saga (/cross-swap)

Composes the MultX bridge with the Lithoswap DEX so a user can go from token A
on chain X to token B on chain Y. The DEX is on Makalu and the bridge locks only
on the Lithosphere chains, so a full route is up to three legs:

bridge_in (X → Makalu) → swap (A → B on Makalu) → bridge_out (Makalu → Y)

Legs whose chain is already Makalu are dropped, so Kamet→Makalu is 2 legs,
Makalu→Kamet is 2, Kamet→Sepolia is 3, etc.

• lib/crossSwap.ts — a persisted state machine: route planner, saga model,
  localStorage persistence, and step executors that reuse lib/bridge.ts
  (lock → wait for validator signatures → claim) and lib/swap.ts (quote →
  approve → swap). currentStep() recomputes the next action purely from the
  saved record, so a user who closes the tab mid-flow resumes exactly where they
  left off. The bridge-out amount is the actual swap output, captured as the
  on-chain balance delta of the received token. External-chain destinations are
  released by the relayer (no user claim); Lithosphere destinations are claimed.
• pages/cross-swap.tsx — setup form (token/chain in + out, amount, slippage,
  live quote preview + route), then a guided step-runner: a progress rail, a
  single context-aware Continue button, chain-switch gating per leg, and polling
  that auto-unlocks claims once validators sign and auto-completes external
  releases. Env-gated by NEXT_PUBLIC_SWAP_ROUTER (same as /swap) — shows a
  "not live yet" state until the DEX is deployed.
• /swap gains a link to /cross-swap.

Verified: tsc clean (aside from pre-existing build-info.test.ts) and
next build compiles /cross-swap as a static route.

Note: this is the orchestration + UX. It exercises the same live bridge and
(once deployed) DEX; end-to-end validation on-chain needs the DEX deployed and
pools seeded.