server: LIGHT_NODE_PEERS — override the seed peers (relay-fleet spoke)

[dkijania]

Lets a node dial a chosen peer set instead of the network's published seeds, keeping the network's chain_id. Paired with LIGHT_NODE_STATIC_PEERS=1, this pins a node to a trusted relay fleet — the wallet/exchange spoke in #17 — and never dials the public seeds. It's the piece #17 flagged as "the missing piece that makes static-peer mode usable by an external customer."

Change

• LIGHT_NODE_PEERS = comma-separated multiaddrs. Parsed by parse_peer_override (trims entries, drops blanks; an all-blank value falls back to the network seeds), then leaked to 'static once at startup so the whole peer plumbing (gossip / sync-ledger reads / broadcast) stays borrow-free and unchanged. Multiaddr validity is still checked at dial time.
• Usage for a fleet spoke: MINA_NETWORK=devnet LIGHT_NODE_PEERS=<hub1,hub2> LIGHT_NODE_STATIC_PEERS=1.

Validation

• 3 hermetic unit tests for parse_peer_override (split/trim, drop-blank entries, all-blank → None). clippy -D warnings + fmt clean; 14 bin tests pass.
• The static-peer connect behavior this feeds (no discovery, pin to the configured set) is already validated on the lightnet by gossip_relay / submit_mempool.
• A live devnet override smoke test is deferred to CI / a clean environment — local runtime runs were blocked by shell/background flakiness this session (the binary itself is healthy: it starts and panics-with-message correctly foreground).

Follow-ups (still #17)

Multiple hubs + health-based failover; fold the subscribe_gossip network knobs into a GossipConfig struct; generalize persistent re-dial to discovery mode.

Refs #17