Extract the trailing-window rate/count implementation into a new src/rate.rs,
shared by PromQL's rate() (engine::evaluate_promql, via a thin i64<->Timestamp
adapter) and alert Threshold/Ratio conditions (alerts::evaluate_condition, via
the new incremental RateWindow), replacing alerts' bespoke VecDeque+prune loop.
Both now agree on what "windowed rate" means for the same data, verified by a
new regression test (engine::tests::dashboard_rate_and_alert_threshold_agree_on_same_windowed_data).
Behavior change: alert Threshold/Ratio windows are now backfilled once, on
first evaluation, from already-ingested logs within their own trailing window
(bounded cost, not a full history rescan) - a rule created mid-session, or a
process that replays existing log content (e.g. --file tailing, which rereads
the whole file from the start on every process start), no longer starts with
a silently empty window. A bare restart with no replayed history (e.g. stdin
ingestion) still starts empty, since there is genuinely no log history to
backfill from in that case - alert state itself remains in-memory only.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
