feat(kvstore): decay eviction freshness and split hit half-life by role

[unsaltedbutter-ai]

KV disk cache evicts small, frequently-reused prefixes before large single-use checkpoints

Under budget-pressure eviction, ds4_kvstore_entry_eviction_score drops small cold prefixes reused across many sessions and keeps large multi-turn checkpoints that were hit 0-1 times and will not be resumed.

Seen after lowering DS4_KV_DISK_SPACE_MB from 256 GiB to 32 GiB. The boot eviction removed every reused prefix and kept ~25 one-shot conversation snapshots:
evicted disk-cache-full tokens=533 hits=16 size=29.86 MiB (reused prefix)
evicted disk-cache-full tokens=564 hits=16 size=30.27 MiB
kept ... tokens=91920 hits=1 size=1229.8 MiB (one-shot chat)
Two effects combine:

1. The hit count decays with a 6h half-life on last_used. A prefix reused on a daily or weekly cadence has its hit evidence erased between uses, so at eviction time a 16-hit prefix scores like a 0-hit file.
2. score = (effective_hits + 1) * tokens/file_size. The constant +1 gives every entry a permanent floor proportional to token density. Large checkpoints are denser (~14 KiB/token vs ~50 KiB/token for a short prefix), so once a prefix's hits decay it ranks below big idle checkpoints no matter how often it was reused.

Net: at a tight budget the cache keeps low-value large one-shot conversations even if quite old and evicts often-but-infrequently reused small prefixes.

Two changes to ds4_kvstore_entry_eviction_score:

• Replace the constant +1 score floor with a freshness term that decays with idle time (8h half-life). A checkpoint that stops being reused loses its density-driven score within about a day instead of riding its raw size forever.
• Decay the hit count on a per-role clock: cold session prefixes, reused across sessions, use a 21-day half-life; conversation tips keep the existing 6h. Only cold is treated as an entry anchor now, so evict/shutdown tips no longer get the 2x anchor prior.

Effect: a large one-shot conversation stays protected inside its resume window, then falls below small, frequently-reused prefixes once idle. The manual pin (future last_used) and the superseded-prefix penalty are unchanged.

Replaying an actual purge with the new scoring keeps the reused prefixes that were evicted at every budget and idle-age tested. Half-lives are #defines; the values are logged in the KV disk cache banner.

Related to #444
Orthogonal to #448