Add tickets to improve metric abstractions

tickets/040-unify-rate-window-implementation.md

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+# 040 — One rate/window implementation, shared by PromQL and alerts
+
+## Summary
+
+"Rate over a window" is implemented twice with different semantics and no
+shared code:
+
+- PromQL's `rate()` (`windowed_rate_series`, `src/engine.rs`) scans the
+  trailing window at every historical point over indexed bucket data
+  (occurrences-per-second, backfilled).
+- Alerts' `Threshold`/`Ratio` conditions (`src/alerts.rs`) maintain an
+  independent live sliding `VecDeque` window over raw log-line matches
+  (current count only, resets on process restart, no backfill), deliberately
+  bypassing the bucket/PromQL pipeline for cost reasons (module doc: a
+  `metric[param]` comparison would require a full bucket rescan per line).
+
+These can and will disagree for equivalent-looking rate/ratio rules, since
+they read different data and compute the window differently.
+
+## Scope
+
+- Extract a single windowed-rate function, e.g. in a new `src/rate.rs`:
+  `fn rate_at(points: &[(Timestamp, f64)], now: Timestamp, window_ms: i64) -> f64`.
+- Have PromQL's `rate()` evaluation call this shared function (behavior
+  should be unchanged for existing dashboard queries).
+- Have alerts call the same function against the bucket store instead of
+  maintaining a separate `VecDeque` over raw matches. If alerts' proportional
+  per-line cost guarantee can't be met by reading buckets directly, solve
+  that inside the shared module (e.g. an incrementally maintained window
+  index) rather than keeping two implementations.
+- As a consequence, alert rate/ratio conditions gain backfilled history
+  (they no longer reset to empty on process restart) — call this out in
+  release notes since it changes alert-firing behavior on restart.
+
+## Acceptance criteria
+
+- Only one windowed-rate implementation exists in the codebase.
+- A dashboard `rate()` chart and an alert `Threshold`/`Ratio` condition over
+  the same metric/window agree on the computed rate for the same historical
+  data, verified with a seeded fixture.
+- Alert conditions no longer silently reset their window on process restart
+  (or this is an explicitly flagged, deliberate behavior change).
+- Existing dashboard `rate()` queries produce unchanged output for existing
+  test fixtures.
+
+## Depends on
+
+None — independent of the param unification work
+([037](037-unify-param-declarations.md)–[039](039-metric-param-query-resolves-declared-params.md)).