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/alerts/episodes.py

@@ -15,7 +15,7 @@ QUIET = frozenset({"inactive", "resolved"})
 # The worst thing an episode went through, most severe first.
 _SEVERITY = ("firing", "acknowledged", "no_data", "pending")
 _WORDS = {">": "above", ">=": "at or above", "<": "below", "<=": "at or below",
-          "==": "at", "!=": "other than"}
+          "==": "at", "!=": "other than", "rose": "on a rise of"}
 
 
 def fires_when(rule) -> str: