fix(gossip): chunk the digest + fail-soft obfuscate — no panic at large mesh scale (#44)

[vxfemboy]

Closes #44.

The bug

Membership::tick_digest built one GossipMsg::Digest from the entire directory with no per-datagram cap. At NodeId(16)+seq(8)=24 bytes/entry, a directory of ≥~2731 members produced an encoded body >65535 bytes; when that gossip datagram was obfuscated (obf_psk set), yip_obf::obfuscate's u16::try_from(body.len()).expect("body fits u16") panicked the daemon. (A full-directory digest also exceeds MTU well before that, so gossip couldn't converge past ~50 members anyway.)

The fix (both options the issue lists)

1. Chunk the digest — tick_digest now returns Vec<GossipMsg>, splitting the directory into chunks of at most MAX_GOSSIP_DIGEST_ENTRIES = 40 entries each (40 × 24 B = 960 B + framing/obf/IP-UDP overhead stays under OBF_MTU_BUDGET = 1200, and 40 ≪ 2731 so the u16 length can never overflow again). This mirrors the existing Records reply cap (MAX_GOSSIP_RECORDS_PER_REPLY). The sole caller loops, sending one datagram per chunk to every gossip target exactly as before. This fixes both the panic and convergence at scale. No wire-format change — a receiver can't distinguish a chunked series from N separate digests.

2. obfuscate is now fail-soft — returns Option<Vec<u8>> (None when the body can't fit the u16 length field) instead of panicking. A wire-facing daemon function should never crash the process; every caller handles None (the egress obf-wrap drops an unshippable datagram via retain_mut rather than sending it empty). Defense-in-depth: after the chunking fix no gossip body approaches the limit, but the panic path is now closed for any caller.

Testing

• obfuscate_is_fail_soft_on_oversized_body: a 65536-byte body returns None, a normal body returns Some (RED before the fix).
• tick_digest_chunks_large_directory: a directory > the cap yields multiple Digests, each ≤ MAX_GOSSIP_DIGEST_ENTRIES, whose union is every node_id exactly once; the single-chunk / empty-directory / debounce cases are also covered.
• cargo clippy --workspace -D warnings clean; tests green: yip-obf 10, yipd 239, yip-rendezvous 21.

The obfuscate signature change rippled to bin/yip-rendezvous's own RDV-envelope callers (small fixed bodies), updated to match.