Use CollectionLength for the persisted event queue length

[cryptokvc]

Fixes #1138.

EventQueueSerWrapper wrote the queue length as u16, while still writing every event, so EventQueueDeserWrapper restored a truncated queue: 65,536 persisted events came back empty and 65,540 came back as 4.

This switches both wrappers to lightning::util::ser::CollectionLength, as suggested in the issue:

• counts below 65,535 keep the existing two-byte encoding, so existing snapshots decode unchanged;
• larger counts use the 0xffff marker followed by the remainder;
• as noted in the issue, this assumes no existing snapshot holds exactly 65,535 events;
• the reader bounds its preallocation to 1,024 entries, since the length comes from persisted data.

Tests (in event::tests):

• event_queue_round_trips_around_u16_max: full round trip at 65,534, 65,535, 65,536 and 65,540 events. It fails on main (65,536 events decode as 0) and passes with this change;
• event_queue_keeps_legacy_encoding_below_u16_max: a small queue encodes to the same bytes as the legacy u16 format and decodes from them.

cargo test --lib passes locally (203 tests); I didn't run the integration tests, which need bitcoind/electrs.

AI disclosure: written with AI assistance (Claude); I reviewed the change and ran the tests above.

🤖 Generated with Claude Code

[ldk-reviews-bot]

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