Stabilize GridLayout interactions and component coverage

[heswell]

Summary

• Replays the gridlayout-tabs foundation onto current main, including the compatibility fixes needed for the current tabs APIs.
• Adds a declarative GridModel scenario harness with 22 passing scenarios and 2 explicit todos after stabilization.
• Completes Playwright component coverage with 16 passing tests and 3 explicit fixmes.
• Adds 13 focused GridLayout showcase examples plus an interactive inspector for tracks, item areas, placeholders/stack state, and serialized JSON.
• Stabilizes GridLayout behavior by enforcing splitter minimum sizes, correcting fractional and multi-track resize math, normalizing irregular removal with multiple unused lines, enforcing non-resizable split/drop rules, and isolating nested GridLayout drag contexts.

Validation

• npx vitest run packages/grid-layout/test/GridModel.scenarios.test.ts --reporter=dot — 22 passed, 2 todo
• PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH='/Applications/Google Chrome.app/Contents/MacOS/Google Chrome' npx playwright test --config=playwright-ct.config.ts packages/grid-layout/src/__tests__/__component__/GridLayout.playwright.test.tsx --project=chromium --reporter=line — 16 passed, 3 fixme
• Targeted npx biome check --diagnostic-level=error ... — passed
• npm run build --workspace @heswell/grid-layout — passed
• npm run build:worker and npm run typecheck — passed
• npm run showcase:prod — production bundle built and the GridLayout route served successfully

Remaining fixmes

1. Palette template drop onto an empty placeholder
2. Palette template centre replacement of existing content
3. Tab reorder by dragging within a tabstrip