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fix: implement ResetIterator on emptyPostingsIterator + assert iterator interfaces

Verifying the reviewer's interface-completeness concern surfaced that
emptyPostingsIterator implemented segment.PostingsIterator and
OptimizablePostingsIterator but not ResetablePostingsIterator (missing
ResetIterator) — the only one of the three iterator types that can share an
optimized termFieldReader's per-segment slice to lack it.

It is harmless today (the reset loop calls ResetIterator via an optional type
assertion and the empty iterator is stateless), but it is an inconsistency.
Add a no-op ResetIterator, and add compile-time assertions that all three
iterator types (empty, unadorned bitmap, unadorned 1-hit) satisfy both
OptimizablePostingsIterator and ResetablePostingsIterator, so a future missing
method fails the build instead of silently disabling an optimization or a reset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>