fix(conversations): initialize LastRound so fresh conversations survive cleanup

[MorquinDevlar]

The bug

getConversation has a 2% chance per call of running maintenance that deletes any conversation more than 10 rounds stale, measured as rNow - LastRound. But AttemptConversation never set LastRound, and the round counter starts at 1,314,000 - so every freshly created conversation looked ancient until its first NextActions call. Any lookup that won the maintenance roll deleted a conversation that was just created.

Two consequences:

• In game: a conversation can silently vanish between creation and its first line of dialogue (2% per lookup in that window).
• In CI: TestAttemptConversation_UsesPluginFile creates a conversation and immediately looks it up, so it fails on ~2% of runs regardless of what a PR changes. Example: the initial Go Tests failure on perf(users): sync the user index incrementally, carry character names in index records #639 (this run), which passed untouched on rerun. Reproducible on master with go test -run TestAttemptConversation_UsesPluginFile -count 300 ./internal/conversations/.

The fix

LastRound now starts at the current round, matching StartRound. Dialogue timing is unchanged: conversations are created by the mob converse command during a round, and actions are pulled by the NewRound_IdleMobs hook on the following round, so the same-round guard in NextActions never bit in the real flow.

Includes a deterministic regression test. Verified with 300 consecutive -count runs, which reliably failed on master before the fix.