fix(cache): improve hybrid attention snapshot eviction - #1550
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Hybrid attention prefix traversal currently refreshes every ancestor snapshot's LRU age, even when only the deepest snapshot is restored. Under cache pressure, this can retain unused historical checkpoints at the expense of other conversations. KV eviction can also stop after removing a tail checkpoint while leaving an unusable KV-only chain allocated.
This change gives small-page snapshots a separate usage timestamp, refreshed after actual state restoration, and reclaims the resulting unreferenced KV-only tail after eviction. Cleanup stops at the root, a referenced prefix, a branch, or a valid small-/big-page checkpoint. Chinese comments explain the lifecycle and eviction boundaries.
Validation: 13 focused CPU control-flow tests passed during development, covering snapshot recency, restore/fallback paths, reference accounting, and cleanup boundaries. The temporary test file is not included in this PR. Flake8 and
git diff --checkpassed. GPU inference and multi-turn performance have not been measured.