Sum AggregationType ingest + CMS per-item estimate(key) - #355

asap_otel_proto: add SumAgg/SumAggDataPoint/SumAggEncoding + Data::SumAgg (sum_agg=18, matching the collector pdata oneof tag). SumAccumulator::from_sum_bytes decodes the self-contained 16-byte Sum payload (float64 sum LE || uint64 count LE) the edge emits — Sum is an aggregation, NOT a sketch, so the payload deliberately does not use the sketchlib envelope proto. otel.rs ingest routes Data::SumAgg through SumAccumulator into the existing ExactAgg(Sum) MetricPoint path (no new SketchKindHandle; sid stays exact_agg:Sum). Cross-language golden test (Go SumWrapper bytes -> sum=100).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

`SketchStore::approx_memory_bytes` is the flusher's evictable-payload gauge
(current_epoch + sealed payloads). At idle it correctly reads ~0 because
payloads are flushed to disk — but the per-sid registry (`instances`
metadata + per-series `InternTable` label caches) stays resident, so the
MEMORY_DIAG line read "0.00 KB" while process RSS sat in the hundreds of MB,
and there was no relief valve for that residue (stale sketch sids are pinned
until config-driven retirement).

Two cohesive changes, both scoped to the SketchStore:

1. Honest memory diagnostic. Adds `InternTable::approx_heap_bytes` and
   `SketchStore::approx_resident_bytes` (registry metadata + intern caches +
   live payload) and `process_resident_bytes()` (/proc/self/statm). The
   30s MEMORY_DIAG line now logs payload (evictable) / registry+intern
   (resident, not flushable) / process RSS, so the real footprint is
   visible. The flusher's payload-pressure trigger is intentionally left on
   the payload gauge — feeding non-flushable registry memory into it would
   livelock the flush loop (it can only evict payload).

2. Idle-sid eviction (opt-in --idle-sid-evict-secs /
   ASAP_IDLE_SID_EVICT_SECS, default 0 = off). A periodic sweep drops the
   in-memory `SidStoreData` (epoch columns + intern cache + series slot) for
   sketch sids that are persistence-backed, write-idle past the threshold,
   AND fully durable on disk (sealed_epochs + current_epoch empty), while
   KEEPING the queryable `SketchInstanceMetadata`. `union_disk_parts_into`
   reads the durable tier via the retained metadata only (never
   `self.series`), so an evicted series stays answerable from disk; the
   append path's `entry().or_insert_with` rehydrates a fresh store on the
   next write. Bounds resident registry memory under series churn. Effective
   idle horizon is max(threshold, persistence_hot_window) since eviction
   waits for a sid's windows to seal+flush first.

Adds `SidStoreData::last_write_unix_ms`, stamped under the append write lock
the hot path already holds (no extra cost). 5 unit tests cover resident
accounting and the eviction predicate / rehydration / durability guard.
Validated live on the Google-cluster trace: 31.5k stale sids reclaimed,
in-memory sid state held to a few thousand vs pinning all 36k; MEMORY_DIAG
now shows payload≈KB / registry≈MB / RSS≈MB instead of a misleading 0.00 KB.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>