ben-manes / caffeine / #5676 / 1

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Materialize the clairvoyant look-ahead to a shared file

opt.Clairvoyant and admission.Clairvoyant each held the whole trace in
memory to compute Belady's next-access times. Materialize them once, up
front, to a fixed-width temp file that every clairvoyant consumer shares
via a sequential cursor, so only the resident set stays on the heap. The
pass also freezes a synthetic trace so the admitter replays the same
sequence the policies see; it previously re-read the trace and threw on
synthetic input.

A forward pass appends the records and a backward pass fills each
next-access pointer from a nextSeen map, keeping the I/O sequential and
the heap at O(distinct keys); a key-only trace is read straight from its
LongStream with no per-event boxing. The reader reaches the policies
through a ScopedValue bound only while they are constructed.

On ARC's DS1 at 4M it runs ~2x faster than the in-memory version and
fits ~4x less heap (512 MB vs 2 GB), bit-for-bit identical.

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