[2269] ui-chart clones its whole series on every message

[n-lark]

Description

Charts copied their entire history on every incoming message just to check it existed and to count points, so the longer a chart got, the slower every update became. This removes those copies. Nothing changes in what charts store or show.

Performance

The store work each chart message does is 95–98% lower. Measured on a chart with 4 lines of 1000 points receiving 16 messages a second (median of 7 runs):

X-Axis Limit	Per message, main	Per message, this PR	Gain
By points (e.g. "OR 1000 points")	5.13 ms	0.25 ms	~20× faster
By time only (default, "last 1 hour")	2.43 ms	0.05 ms	~48× faster

At 16 messages a second that is about 82 ms of store work every second down to 4 ms when limited by points, and 39 ms down to under 1 ms when limited by time. The rest of the chart's per-message work is unchanged.

Test plan

• has and keepLatestPerTopic make no cloneMessage calls
• keepLatestPerTopic keeps the newest N per topic in order, treats a missing topic as one series, leaves the series alone under the limit, and ignores a widget holding a single message
• get still returns a copy
• Matches the removed ui_chart logic on 5,000 random series, including missing, empty and numeric topics
• Line chart limited to 5 points with two lines: 5 per line in the store, 5 per line drawn live and after a reload, no console errors

Related Issue(s)

Closes #2269

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label