fix(event): keep a preset created_at in GenericEvent.update() (2.4.2)

[tcheeric]

Summary

GenericEvent.update() restamped created_at with the clock before computing the id. A caller that set created_at, called update() and signed got an id for a different second than the one it saw, so relays rejected the event as invalid: bad event id. NIP-59 randomised timestamps were also replaced by the send time. NIP-01 derives the id from the event's own fields, so this aligns update() with the protocol.

Closes #559

Type of change

• fix - Bug fix (non-breaking)
• docs - Documentation only
• chore - Build, CI, or tooling changes

What changed?

• GenericEvent.update() keeps a created_at that is already set and only stamps the clock when it is unset (null or 0).
• New GenericEvent.updateWithCurrentTime() for deliberate restamping.
• Regression tests in GenericEventUpdateTest, including the NIP-01 id check suggested in the issue.
• MCP server how-to: "See it in use" section pointing to the Lyrebird bot.
• Release bump to 2.4.2 across all modules, plus CHANGELOG.

Testing

• Unit tests pass: mvn -q clean install -DskipITs (exit 0)
• Integration tests pass: mvn -q verify has one failure, NostrMcpServerStdioIT.aHostCanSeeWhichIdentitiesTheServerHolds. It also fails on the commit before this fix: the test expects an empty keystore but reads the local OS keychain, which holds an identity. Unrelated to this change.
• End to end: set created_at, update(), waited 1.1s, signed with a real Identity. created_at kept, id unchanged through signing, id equals the NIP-01 hash, Schnorr signature verifies.

Checklist

• PR title follows conventional commits
• Changes are focused and under 300 lines
• Tests added/updated for new functionality
• No new compiler warnings introduced
• CHANGELOG.md updated