docs: ADR 0004, Playground glossary and research

[mnaimfaizy]

Summary

Documentation only; no application code changes.

• ADR 0004 (accepted): Playground traces share the story snapshot and the 2D/3D views, are recorded by AST instrumentation on the browser's own engine, and are never presented as stories. There is exactly one Playground, it is dark, and work is JavaScript-first. The old src/components/playground/ is to be retired.
• CONTEXT.md: adds the Playground vocabulary (Playground, Trace, Step, Snapshot, Lens, Console, Example, Exercise), the story terms that were in use but undefined (Region, Entity, Cell, Slot, Reference, Focus, Timeline), the Marble diagram tool, and renames Step model to Beat model. Shot and Transcript are updated to match the code.
• AGENTS.md: a Playground rules section and the dark-theme exception to "Light mode only". .github/copilot-instructions.md is regenerated by npm run agents:sync.
• Research: the October 2026 playground audit (what happened when the Playground was run, and why) and the playground design research with sources.

Known gaps after this PR

• The glossary says "Beat model", while AGENTS.md, the step-model skill, ADR 0001, the 3D standard and the code still say "step model". The rename is a separate pass.
• The new Playground rules describe the target. The current Playground code does not follow them yet (its lenses still use React Flow).
• The old playground is still mounted on the Big-O and Data Structures pages.

Checks

• npm run agents:check passes.
• tsc, lint, tests and build were not run locally, since no code changed.

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