Prepare greyUI 0.6.0 container-aware window release

[elithrar]

Summary

• make Window chrome and Header/Field action layouts container-aware, with explicit auto/floating/stacked and auto/inline/stacked APIs
• unify Window frame geometry across active, inactive, collapsed, menu, content, and status rows
• add coordinated Base UI menubar semantics, complete Menu parts, popup positioning overrides, and collision clamping
• make Select, Combobox, and Autocomplete popups anchor-sized by default with explicit content-width overrides and item parts
• prevent modal overlays—including context menus—from shifting the docs layout by reserving the root scrollbar gutter
• add permanent docs regression fixtures plus Playwright geometry checks at 1280, 768, 390, and 320 px
• prepare package, docs, changelog, and release automation for 0.6.0

Review loop

Six implementation tracks were independently reviewed against the original 0.6 plan. Follow-up findings around collapsed container sizing, frame seam ownership, disabled menu styling, popup clamping, explicit layout fixtures, edge collision coverage, and pre-tag docs validation were fixed and re-reviewed with no remaining implementation blockers.

A final test review removed or consolidated 33 redundant unit tests that primarily exercised React or Base UI behavior. The retained suite targets greyUI-owned APIs, layout and state contracts, accessibility defaults, release safety, and named regressions.

Validation

• npm run check — 51 unit tests passed
• npm run test:browser — 7 Playwright browser tests passed
• npm run build
• npm run build:docs
• npm run pack:check
• npm run verify:docs
• PR CI run 173 passed in full

Release

Do not create v0.6.0 manually. After this PR is merged, the release workflow validates the merge commit, creates v0.6.0, reruns validation on the tag, and publishes greyui@0.6.0 with provenance.