feat(alerts): deterministic metric-transition reducer with manager parity harness

Phase 1 of docs/ALERT_ENGINE_EVOLUTION.md. internal/alerts/reducer is a
pure transition core for the metric-threshold family — hysteresis
trigger/clear, sustained-for delay with dip reset, warning/critical
severity derivation with the percentage 99-cap — characterized from
Manager.checkMetric, with time entering only through the signal's
ObservedAt so every sequence is deterministic and replayable.

reducer_parity_test.go drives the live manager and the reducer through
identical observation sequences (simulated time via the established
pending-backdate trick) and fails on any divergence after every step,
with the manager as the reference. The harness proved itself on its
first run by catching the stale-resolve defect fixed in the previous
commit.

Registers coverage gap alert-lifecycle-contract-coverage. Remaining
slices: offline/confirmation families, ack lifecycle, intent
interaction, shadow-mode runtime feed.

docs/release-control/v6/internal/records/alert-lifecycle-contract-coverage-2026-08-26.md

@@ -0,0 +1,38 @@
+# Alert lifecycle transitions lack a deterministic contract harness
+
+Recorded: 2026-08-26
+Origin: alerting audit (2026-08-26) finding 2 and docs/ALERT_ENGINE_EVOLUTION.md
+Phase 1.
+
+## Gap
+
+The alert manager's transition semantics (hysteresis trigger/clear,
+sustained-for delays, severity derivation, confirmation counts) are
+implemented imperatively across per-family check paths and asserted only
+by per-unit tests. There is no deterministic, engine-independent statement
+of the lifecycle contract, so regressions in the frozen transition core
+keep shipping (#1682, #1683, #1553, #1693, #1724) and cross-family
+semantic drift is invisible.
+
+## Resolution
+
+Phase 1 of docs/ALERT_ENGINE_EVOLUTION.md: a pure transition reducer
+(`internal/alerts/reducer`) that characterizes the manager's semantics
+family by family, plus a parity harness (`reducer_parity_test.go` in
+`internal/alerts`) that drives both engines through identical observation
+sequences and fails on any divergence. The reducer must characterize, not
+improve: divergences are reducer bugs unless investigation proves a
+manager defect, which is then fixed in the manager first.
+
+First slice (2026-08-26): the metric-threshold family. The harness
+immediately caught a real manager defect: `checkMetric` hysteresis
+resolution removed alerts by the legacy `<resourceID>-<metric>` ID while
+canonical-identity alerts are stored under the canonical state key and the
+legacy ID carries no alias, so the removal silently no-oped — a resolved
+notification went out while the alert stayed active and re-resolved every
+poll (guest per-disk usage alerts were the remaining production caller;
+user-visible as the stale-alert class, e.g. #1580). Fixed at the caller
+with a focused regression test before the parity slice landed.
+
+Remaining for later slices: offline/confirmation families, ack/snooze
+lifecycle, intent-policy interaction, and a shadow-mode runtime feed.

docs/release-control/v6/internal/status.json

@@ -1,6 +1,6 @@
 {
   "version": "6.0",
-  "updated_at": "2026-08-08",
+  "updated_at": "2026-08-26",
   "scope": {
     "active_repos": [
       "pulse",
@@ -9787,10 +9787,52 @@
           "kind": "file"
         }
       ]
+    },
+    {
+      "id": "alert-lifecycle-contract-coverage",
+      "summary": "Alert transition semantics (hysteresis, sustained-for delays, severity derivation, confirmations) have no deterministic engine-independent contract harness, so transition-core regressions ship undetected (#1682, #1683, #1553, #1693, #1724) and cross-family drift is invisible. Resolution is Phase 1 of docs/ALERT_ENGINE_EVOLUTION.md: a pure characterization reducer (internal/alerts/reducer) plus a parity harness driving reducer and manager through identical sequences; the first metric-family slice already surfaced and fixed a silent stale-resolve defect in checkMetric.",
+      "owner": "project-owner",
+      "status": "triaged",
+      "recorded_at": "2026-08-26",
+      "lane_ids": [
+        "L6",
+        "L13"
+      ],
+      "subsystem_ids": [
+        "alerts"
+      ],
+      "proposed_resolution": "lane-expansion",
+      "coverage_impact": 5,
+      "evidence": [
+        {
+          "repo": "pulse",
+          "path": "docs/release-control/v6/internal/records/alert-lifecycle-contract-coverage-2026-08-26.md",
+          "kind": "file"
+        },
+        {
+          "repo": "pulse",
+          "path": "docs/ALERT_ENGINE_EVOLUTION.md",
+          "kind": "file"
+        }
+      ]
     }
   ],
   "candidate_lanes": [],
-  "work_claims": [],
+  "work_claims": [
+    {
+      "id": "claude-alert-reducer-coverage-gap-alert-lifecycle-contract-coverage",
+      "agent_id": "claude-alert-reducer",
+      "summary": "Phase 1 slice: metric-family reducer + parity harness; includes checkMetric stale-resolve fix",
+      "target_id": "v6-product-lane-expansion",
+      "claimed_at": "2026-08-26T21:52:17Z",
+      "heartbeat_at": "2026-08-26T21:52:17Z",
+      "expires_at": "2026-08-27T01:52:17Z",
+      "work_item": {
+        "kind": "coverage-gap",
+        "id": "alert-lifecycle-contract-coverage"
+      }
+    }
+  ],
   "open_decisions": [],
   "source_of_truth_file": "docs/release-control/v6/internal/SOURCE_OF_TRUTH.md",
   "resolved_decisions": [