…hart marup for test fixtures

Provides a .NET 10 devcontainer with Node LTS and GitHub CLI for running
agents against the repo in an isolated environment. Mounts Claude/Codex
user config from the host.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Splits Chart2D, Chart3D, ChartCarpet, ChartDomain, ChartMap, ChartPolar,
ChartSmith, and ChartTernary monolithic files into individual per-method
files under matching subdirectories. 73 total method files created.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Add a shared chart-baseline-generation harness under tools and point the Claude, Codex, OpenCode, and repo agent guidance at the same workflow. This keeps the skill instructions consistent and avoids duplicating the investigation script per agent.

Cover the remaining split Chart2D wrapper methods with dedicated htmlcodegen tests and record the verification in the API file split plan. This keeps the wrapper split independently validated before continuing with the next Chart API refactor step.

Move the remaining top-level Chart wrapper methods into dedicated partial files and cover the layout-sensitive helpers with focused C# htmlcodegen tests. This keeps the Chart API split consistent before continuing with the 3D and polar wrappers.

Cover the split 3D and polar C# wrappers with focused htmlcodegen tests and record the verification in the API file split plan. This closes the remaining wrapper coverage gap for those families before continuing to map and ternary.

Cover the split map and ternary C# wrappers with focused htmlcodegen tests and record the verification in the API file split plan. This closes the remaining coverage gap for those families before moving on to domain, carpet, and smith.