fix: harden responsive layout integration

[Foscat]

What changed

• wrap the split, two-pane, three-pane, and media specimens in local inline-size containment scopes
• document why the queried primitive must be nested beneath the boundary
• keep Reel's horizontal overflow intentional while making the specimen keyboard-accessible and explicitly scrollable
• pin the published layout-style-css@2.1.1 package and align install, CDN, fixture, metadata, and export contracts
• add direct overflow, zero-width track, local-allocation, recipe-divergence, and runtime-error coverage at 390, 768, 900, 992, 1024, 1240, and 1968 pixels
• replace the vulnerable preview-server dependency with a repository-owned static preview server and refresh patched Next/PostCSS/Sharp tooling

Root cause

Container-responsive primitives had no parent containment boundary matching their allocated atlas cell. A queried element cannot establish and query its own inline-size containment context.

Verification

• npm.cmd run quality
• 32 unit tests
• 10 fixture tests
• 9 export tests
• 118 Playwright tests with tagged Chromium, Firefox, and WebKit coverage
• production dependency audit with zero vulnerabilities
• git diff --check

The full npm audit retains nine dev-only findings in Next's bundled ESLint/minimatch chain; npm's proposed remediation is a breaking ESLint 10 upgrade, and the compatible lint configuration remains green.

[Copilot]

Pull request overview

Hardens the layout laboratory’s container-responsive specimens by ensuring they query within an appropriate inline-size containment boundary, while also aligning published dependency versions and replacing the external preview server used for Playwright runs.

Changes:

• Wrap split/two-pane/three-pane/media specimens in a local container-type: inline-size scope and document the container-query constraint.
• Replace the serve-based Playwright preview server with a repository-owned static preview server and update Playwright webServer command accordingly.
• Pin layout-style-css@2.1.1 everywhere (fixtures, docs, ecosystem metadata, and tests) and expand browser coverage for responsive/overflow/runtime-error assertions across multiple viewports.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/fixtures.test.mjs	Updates expected fixture asset paths and versions for layout-style-css@2.1.1.
tests/export.test.mjs	Aligns structured data + exported asset expectations with layout-style-css@2.1.1.
tests/ecosystem.test.ts	Updates pinned install/CDN strings and registry ordering to include layout-style-css@2.1.1.
tests/browser/site.spec.ts	Updates metadata/version assertions for layout-style-css@2.1.1.
tests/browser/capabilities.spec.ts	Adds containment boundary checks, Reel accessibility/scroll assertions, viewport matrix expansion, and runtime error capture.
scripts/verify-export.mjs	Aligns structured data verification with layout-style-css@2.1.1.
scripts/serve-preview.mjs	Adds a repository-owned static preview server for Playwright’s webServer.
scripts/build-fixtures.mjs	Updates fixture build contracts and expected asset targets to layout-style-css@2.1.1.
README.md	Updates pinned install/CDN examples and package table version to layout-style-css@2.1.1.
playwright.config.ts	Switches Playwright webServer command to use the new preview server script.
package.json	Pins layout-style-css@2.1.1, updates Next/eslint tooling, removes serve, and adds preview:serve.
package-lock.json	Lockfile refresh for dependency updates (Next/PostCSS/Sharp/etc.) and removal of serve.
app/styles/labs.css	Introduces .primitive-query-scope containment boundary styling and documents why it’s needed.
app/data/ecosystem.ts	Updates ecosystem package version + pinned install/CDN links for layout-style-css@2.1.1.
app/components/labs/LayoutLab.tsx	Wraps container-responsive primitives in .primitive-query-scope and adds Reel accessibility attributes.

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[Foscat]

Fixed in d639087. File streams now register the error handler before the open event, return a controlled 500 when opening fails, and close an already-started response without leaving an unhandled stream error. Added a real HTTP regression test that issues two failing reads and verifies the server remains available.