refactor(stage-gate): use format_gate_rates instead of duplicating it (#1432)

Third review round on #1340. `format_gate_rates` had no production caller —
only its own unit tests — while `evolution_metrics.format_health` rendered the
same data inline in a different shape. Two renderings of one thing, one of them
dead, in a repo whose integration skill runs an explicit dead-code check on
every PR.

`format_gate_rates` now emits exactly what the health line body needs and
`format_health` calls it, so the shape lives next to the data that defines it.
Lazily imported for the same reason `compute_health` imports the module lazily:
the evolution package is not always available, and the health line must still
render without it.

The output is byte-identical to before — verified end to end rather than by
reading:
  healthy   ...effort_budget=3.0 gate[local_triage]=refine 50%/restart 0% (n=4) | healthy
            -> check_health: 0 alerts
  mis-tuned ...gate[local_triage]=refine 0%/restart 75% (n=4) | HIGH_STAGE_RESTART_RATE:local_triage=75%
            -> check_health: 1 alert

Added a test asserting the rendered segment contains no `|`, since the health
line's flags must remain the last pipe-separated segment for
`evolution_watchdog.check_health` — that constraint was previously only implied
by a comment.

52 passing in test_stage_gate.py + test_evolution_metrics.py; ruff clean.

Refs #1340

Author: Lexus2016

evolution/lib/stage_gate.py

@@ -290,12 +290,21 @@ def gate_flags(
 
 
 def format_gate_rates(rates: dict[str, dict[str, Any]]) -> str:
-    """One-line summary per boundary for the evolution-health sidecar."""
+    """Per-boundary rates as they appear in the evolution-health line body.
+
+    Emitted WITHOUT a leading marker and with no trailing ``|``: the health
+    line's flags must stay the last segment after the final pipe, because
+    ``evolution_watchdog.check_health`` keys on the line ending in
+    ``| healthy``. This belongs beside ``effort_budget`` in the body, under the
+    same constraint.
+
+    Returns ``""`` for an empty mapping so the caller can concatenate it
+    unconditionally.
+    """
     if not rates:
         return ""
-    parts = [
-        f"{stage}(n={b['total']} refine={b['stage_refine_rate']:.0%} "
-        f"restart={b['stage_restart_rate']:.0%})"
+    return " ".join(
+        f"gate[{stage}]=refine {b['stage_refine_rate']:.0%}/"
+        f"restart {b['stage_restart_rate']:.0%} (n={b['total']})"
         for stage, b in sorted(rates.items())
-    ]
-    return "[stage-gate] " + " ".join(parts)
+    )

evolution/tests/test_stage_gate.py

@@ -224,5 +224,15 @@ def test_format_names_each_boundary(self):
         from evolution.lib.stage_gate import compute_gate_rates, format_gate_rates
 
         out = format_gate_rates(compute_gate_rates(self._recs(ACCEPT, REFINE)))
-        assert "[stage-gate]" in out
-        assert "local_triage" in out
+        assert "gate[local_triage]" in out
+        assert "refine 50%" in out
+        assert "restart 0%" in out
+        assert "(n=2)" in out
+
+    def test_format_carries_no_pipe(self):
+        """The health line's flags must remain the last `|`-separated segment —
+        `evolution_watchdog.check_health` keys on it ending in `| healthy`."""
+        from evolution.lib.stage_gate import compute_gate_rates, format_gate_rates
+
+        out = format_gate_rates(compute_gate_rates(self._recs(ACCEPT, RESTART)))
+        assert "|" not in out

scripts/evolution_metrics.py

@@ -209,12 +209,17 @@ def format_health(h: Dict[str, Any]) -> str:
     gate = ""
     rates = h.get("stage_gate_rates") or {}
     if rates:
-        # Same rule as effort_budget: body only, never the tail (#1340).
-        gate = " " + " ".join(
-            f"gate[{stage}]=refine {b['stage_refine_rate']:.0%}/"
-            f"restart {b['stage_restart_rate']:.0%} (n={b['total']})"
-            for stage, b in sorted(rates.items())
-        )
+        # Rendered by stage_gate's own formatter so the shape lives next to the
+        # data that defines it. Same rule as effort_budget: body, never the
+        # tail (#1340). Lazily imported for the same reason compute_health
+        # imports it lazily — the evolution package is not always available.
+        try:
+            from evolution.lib.stage_gate import format_gate_rates
+
+            rendered = format_gate_rates(rates)
+            gate = f" {rendered}" if rendered else ""
+        except ImportError:
+            gate = ""
     return (
         f"[evolution-metrics] {h['cycles_active']}/{h['cycles_total']} active cycles: "
         f"success={_pct(h['cycle_success_rate'])} "