Audit Md11 (part 2 of 2). Adds an explicit Clear() so BattleSession can
release the archive at battle-end instead of waiting for the participant to
be GC'd. _next is intentionally NOT reset — a post-Clear emit is a bug per
the design, but the seq stream must stay monotonic if it does happen.
Tests cover empty archive after Clear, _next preservation across Clear,
and Clear-on-empty no-op. The BattleSession integration that calls Clear
lands in the next commit.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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