[n-lark]

Description

This is a refactor that moves utilities into their own files, so reactive.js stops growing, and moves chart series into a class:

• a chart's points, stored messages and history live in one object, with the rules that keep them in step next to them
• state sits in named fields instead of closures passed between functions, so checks like "is this proxy still current" are a method you can read
• flow writes to charts (next PR) need per-chart state and their own proxy traps, which fit on this class

No new functionality or behavior changes. Focused on readability and preparing for flow writes.

Review tip: git diff --color-moved shows the moves as moves. Commits 1 and 2 are moves; commit 3 is the class.

Test plan

• New specs pin behaviorr that already held and the class must keep: history across value/chart switches, an old value reference going quiet, a failed copy leaving the key unchanged, a datapoint with 200,000 points
• 1,500 seeded random runs (about 225,000 operations) give identical results, onChange logs and errors on the old and new store
• Breaking each new seam fails a spec
• Chart benchmarks within noise of feat: [2322] Dashboard data store - Chart performance #2324, except the 1,000-point time window (about 44 → 50 µs), a V8 inlining difference in the benchmark loop rather than extra work

Related Issue(s)

Part of #2322. Based on #2324.

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