fix+feat: unbreak CI + add Across V3 cross-chain decoder (Arbitrum-preferring)

[abhicris]

Root cause of CI break

cargo clippy -- -D warnings rejected six things the initial scaffold left behind:

1. Unused ResolverError import in src/solver/engine.rs.
2. Unused raw: &[u8] parameter in UniswapXDecoder::decode (stub impl).
3. Dead fields on UniswapXOrder (order_status, swapper) and on OrderInput (end_amount) — these are real API fields we'll want for audit/risk/Dutch-decay work, so they're kept with #[allow(dead_code)] and a comment explaining why, rather than deleted.
4. clippy::implicit_saturating_sub in Intent::time_remaining — replaced with saturating_sub.
5. clippy::let_and_return on the surplus_f64 binding in SolverEngine::evaluate — inlined.
6. rustfmt drift (import ordering, block bracing). Fixed by running cargo fmt.

All fixes are surgical — no behaviour changes.

What's new

src/intents/across.rs — Across Protocol V3 decoder that the README promised as "Planned". It:

• Implements IntentDecoder, so it drops into the existing SolverEngine next to UniswapX.
• Exposes decode_deposit_event — a pure, network-free parse of a V3 FundsDeposited-shaped JSON payload into our normalised Intent. That's what the engine and tests both go through.
• Exposes RoutingPreferences with preferred_dest: Chain::Arbitrum as the default. Solvers that want to prefer Base or Optimism can .with_preferences(...).
• Ships with 5 offline unit tests (ETH→Arbitrum decode, default prefers-Arbitrum, custom preference override, unknown-chain rejection, raw-bytes decode() via serde_json).
• Provides a live probe via fetch_open_intents against app.across.to, but returns an empty list on error rather than propagating, so the engine's cycle loop stays healthy when the API flakes.

CLI wired too:

resolver scan  --protocol across --chain ethereum
resolver solve --protocol across --chain arbitrum

Why it matters for the agent-economy thesis

Agents that settle across chains need intent-based bridging with a preferred landing chain. Across V3 is the protocol for that today; Arbitrum is the landing chain for stablecoin-denominated (CR8-USD-style) agent settlement — cheap gas, deep USDC/USDT liquidity, and the ecosystem the AI-agent economy is building on. This decoder is the first step: every Across deposit flowing Ethereum→Arbitrum now becomes a normalised Intent the solver can reason about, with a preference knob that says "this one lands where agents want to be."

Next steps (separate PRs):

• Replace the JSON decode() fallback with real ABI decoding of the V3 event log.
• Teach SolverEngine::evaluate to weight quotes by RoutingPreferences::prefers.
• Add a CoW Protocol decoder (the other "Planned" item in the README).

— kcolbchain / Abhishek Krishna