## Problem
`HeapAlloc`-based eviction cascades on large databases — evicts down to
near-zero packets because Go runtime overhead exceeds `maxMemoryMB` even
with an empty packet store.
## Fix (per Carmack spec on #710)
1. **Self-accounting `trackedBytes`** — running counter maintained on
insert/evict, computed from actual struct sizes. No
`runtime.ReadMemStats`.
2. **High/low watermark hysteresis** (100%/85%) — evict to 85% of
budget, don't re-trigger until 100% crossed again.
3. **25% per-pass safety cap** — never evict more than a quarter of
packets in one cycle.
4. **Oldest-first** — evict from sorted head, O(1) candidate selection.
`maxMemoryMB` now means packet store budget, not total process heap.
Fixes#710
Co-authored-by: you <you@example.com>
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