Fix critical bugs and harden error handling for production readiness

[vincenzopalazzo]

Summary

• P2P graph node2 bug: add_channel used node1 ID for both endpoints, so node2 was never added to the graph — completely breaking P2P topology for probabilistic routing
• LDK route amount/delay calculation: was accumulating fees instead of computing correct forward amounts per hop, producing routes with wrong amounts at every hop
• 5 panic sites removed: replaced unwrap()/expect() with proper error propagation in probabilistic strategy (construct_route_params, convert_route_to_output), plugin on_init, pay handler (cln_rpc_path), and gossip channel conversion
• Error messages fixed: amount validation now distinguishes "both specified" vs "both missing" instead of showing the same confusing message for both
• Random seed: replaced hardcoded [0; 32] with time+payment-based entropy for route randomization
• Clippy cleanup: clone_on_copy, derivable_impls, unused import

Production readiness plan

These fixes address the critical correctness and crash bugs. Remaining work for feature-complete production readiness:

P1 — Must have for real-money testing:

1. Basic retry logic (at least 1 retry on transient WIRE_TEMPORARY_* failures)
2. CLTV safety validation (minimum 18 blocks, overflow check, max bound)
3. Fee bounds (maxfee parameter with hard ceiling enforcement)
4. waitsendpay timeout (currently blocks indefinitely)

P2 — Required for production:
5. Multi-part payments (MPP) — without this, payments > single channel capacity always fail
6. Gossip map caching in plugin state (currently reloads gossip_store on every call)
7. Migrate from deprecated decodepay to decode RPC
8. Implement barqrouteinfo for dry-run route inspection

P3 — Competitive with renepay:
9. Liquidity learning from payment attempt results (Bayesian updates)
10. Payment splitting via MCF optimization
11. Shadow routing for privacy
12. Configurable scoring parameters

Test plan

• cargo build passes
• cargo clippy --all --tests passes (remaining warnings are pre-existing: unused root_path field, unimplemented route_info dead code)
• cargo test --all — 2/3 pass, 1 pre-existing failure (test_rapid_gossip_sync_network_not_empty fails on main too due to empty Testnet gossip data)
• Integration tests require CLN + bitcoind setup (not run in this PR)

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