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/timeline/record.py

@@ -112,7 +112,7 @@ def alert_summary(type_: str, rule, value: float | None) -> str:
     if metric in _BINARY:
         reading = breach = f"{res} is {metric_value(metric, value)}"
     else:
-        op = getattr(rule.operator, "value", rule.operator)
+        op = getattr(rule.operator, "said", rule.operator)
         reading = f"{said} on {res} is {metric_value(metric, value)}"
         breach = f"{reading} ({op} {metric_value(metric, rule.threshold)})"
     return {