feat(alerts): a rule can fire when a reading rises, "rose by N" (#6724)

A rule compared a reading with a fixed number, so Synology Disk 1's bad
sectors going from 3,160 to 3,392 could say nothing: a count that only grows
is soon past any line. "rose by" compares each reading with the one before it,
kept on the alert's state and only for such a rule. A fall (a replaced disk)
doesn't fire. On the rule form, through MCP, and worded in emails, the alert
sentence, history and the timeline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/mail.py

@@ -224,6 +224,8 @@ def alert_facts(alert: dict, tz: tzinfo, show_utc: bool = True) -> dict:
             else _upper_first(f"{counted}{on} again.")
     elif counted:
         headline = _upper_first(f"{counted}{on}.")
+    elif state == "resolved" and alert.get("operator") == "rose":
+        headline = f"{metric_label(metric)}{on} has stopped rising" + (f", at {reading}." if reading else ".")
     elif state == "resolved":
         headline = f"{metric_label(metric)}{on} is back within its limit" + (
             f", at {reading}." if reading else ".")