demux/packet_pool: incrementally free old packets on pop

Until now the pool only released its packets on shutdown or when the
cache size was reduced at runtime, so it kept growing across a session.
The amount of packets would be indirectly bound by demuxer cache size,
else the packets are reused. Still over long playback or switching
files, this may give impression of elevated memory usage, or even memory
leak. This was particularly visible with cache-on-disk, where users
reasonably expect RAM usage to stay low.

Pop now keeps the first POOL_HOT_RESERVE packets at the head of the
freelist for fast reuse, and frees up to POOL_GC_BATCH packets from
beyond that on each call. Bulk pushes (e.g. cache reset on file end)
remain no-work policy. The pool drains across subsequent allocations
rather than stalling on a single huge free. Self-balancing without any
size tracking.

This drains the pool in steady state. Note that on file switch we
effectively reuse only ~N / POOL_GC_BATCH packets from the old cache,
because each pop reuses just one packet while freeing up to
POOL_GC_BATCH. That's fine, both free and alloc calls are throttled to
the same per-pop rate, so allocator pressure stays bounded even though
not all packets get reused.