feat(config): allow idle eviction on single_value per_value_lazy filter fields [GATED — HOLD]

[JustMaier]

⚠️ DRAFT — DO NOT MERGE/ENABLE YET

This is the real fix for the only unbounded term in the prod RSS warm-growth (P2), but its config activation is gated. The code here is inert: it only relaxes a validator; no field gains eviction until a config opts in. See the gate below.

Why

postId / postedToId are single_value per_value_lazy filter fields with no eviction, so their per-value bitmaps accumulate unbounded on a serving pod (postedToId grew 1,833 → 102,663 resident bitmaps in 10h on prod bitdex-0). They are the only genuinely monotonic term in the RSS ratchet. The config validator hard-rejected eviction: on any non-multi_value field, so bounding them required a code change.

What

One change: relax the validator from field_type == MultiValue to MultiValue || per_value_lazy (config.rs), mirroring the exact lazy_value_fields inclusion condition in ConcurrentEngine (concurrent_engine.rs:584). Eager single_value fields (e.g. userId) stay rejected — they load whole and can't reload one evicted value.

Everything else already handles these fields safely:

• the idle sweep already skips DIRTY VBs (concurrent_engine.rs ~3419) — unpersisted activation inserts are never dropped (the load-bearing correctness property);
• eviction_configs and stamp-on-read are not field-type-gated;
• postId/postedToId are already in lazy_value_fields, and every lazy-load apply provably preserves diffs (load_base = base |= disk), so an evicted value re-loads correctly.

Tests

config::tests: accepts eviction on single_value + per_value_lazy, still rejects eager single_value, still accepts multi_value (regression). 14 passed.

🔒 Gate — activate the config ONLY after all three

1. v1.1.46 orphan work lands — enabling postId eviction adds evict+reload churn on postId during the still-open activation window (whole-batch op-loss).
2. fixer's 2a stamp-on-lazy-load fix — the sweep's unwrap_or(true) (concurrent_engine.rs ~3430) currently makes never-query-stamped values evictable once clean. A replay/activation-loaded postId that hasn't been queried would be evicted immediately. B assumes 2a's fixed behavior (a never-stamped value is not evicted before its idle window).
3. Long idle (>= 7200s) for postId/postedToId — each reload is a disk read on the query hot path; short idles like tagIds' 300s would over-evict a high-cardinality FK.

All three are also in the commit message. Convergence note: gate (2) is independently the same prerequisite as fixer's 2a.

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