Add docs for chart bounded context

[sgress454]

Related issue: Resolves #47876

Adds internal documentation for the charts bounded context, including how to add new datasets and how to test. Usable by devs and LLMs.

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[Copilot]

Pull request overview

Adds internal developer documentation for Fleet’s chart bounded context, describing its architecture, data model (SCD type-2 + bitmaps), how to add datasets, and how to test/seed chart data.

Changes:

• Added comprehensive docs for server/chart/ (architecture, dependency rules, write/read paths, adding datasets, testing, dev tooling).
• Added a server/chart/CLAUDE.md pointer/summary for quick orientation.
• Updated tools/charts-collect/README.md to reference host_scd_data and the correct internal constants source file.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tools/charts-collect/README.md	Updates collection docs to reflect current SCD table + internal constants reference.
server/chart/README.md	New end-to-end bounded-context documentation (architecture, data model, extension + testing guidance).
server/chart/CLAUDE.md	New quick “read this first” doc pointing to the main README and key gotchas.

Comments suppressed due to low confidence (1)

tools/charts-collect/README.md:13

• This CVE description mentions closing the "prior-day" row and overwriting "today's" row, but the collector actually keys snapshot rows to hour boundaries (see bucketStart := time.Now().UTC().Truncate(time.Hour) in tools/charts-collect/main.go) and closes the prior open row at the current hour boundary. Updating this avoids misleading readers about the SCD semantics.

  bitmaps, and reconciles into `host_scd_data` (`dataset='cve'`). Unchanged
  CVEs keep their open row; changed bitmaps close the prior-day row and open
  a new one for today; intra-day changes overwrite today's row via ODKU.

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