feat(react-ui): opt-in layered CSS via ./layered-components.css and ./layered/styles/*

[ankit-thesys]

Summary

Makes @layer openui an opt-in instead of the default (revises #589/#621's distribution strategy before it ships in any release):

• ./components.css and ./styles/* return to unlayered 0.11.x behavior — existing consumers see zero change on upgrade. Verified byte-equivalent to the pre-feat(react-ui): adopt CSS cascade layers (@layer openui) via build-side post-process #589 build pipeline.
• New additive exports ./layered-components.css and ./layered/styles/* carry the same rules wrapped in @layer openui for consumers who want plain-CSS overrides (and the Tailwind v4 recipe).
• openui-defaults.css / ThemeProvider runtime injection stay unlayered in both modes (runtime theming contract).
• Removes the package-wide browserslist cascade-layers floor (now only the opt-in path needs it; documented in docs instead).
• Fixes the BOM bug found in pre-release verification: Sass's leading U+FEFF, once wrapped, landed mid-stylesheet and silently killed the first rule (the :root theme tokens). wrapInLayer now strips it — with unit tests (the package's first real test suite; --passWithNoTests removed).
• Adds check-css-artifacts.js to prepublishOnly: default-unlayered / layered-wrapped / BOM-free / defaults-unlayered invariants are now mechanically enforced (guard proven to catch an injected BOM).
• Docs explain both modes; docs app + the nine layer-recipe examples dogfood the layered variant, and multi-agent-chat / vercel-ai-chat are consolidated to a single CSS import site (the fix(css): re-adopt @layer openui (#589) and make the docs honor cascade layers #621 Turbopack hazard). material-ui-chat / fastapi-backend deliberately stay on the unlayered default.

Why

Flipping the default would change override semantics for every existing consumer (unlayered resets suddenly beat component styles — the same silent visual regression that broke our docs site in #620) and would bundle that risk with the react-syntax-highlighter security fix in the next release. Opt-in keeps the next react-ui release a safe patch (0.11.9). Default flip is deferred to 1.0 with a migration window.

Verification

• 6/6 unit tests; pnpm build + pnpm run check:css + pnpm run ci green
• Packed tarball inspected: exports resolve, default CSS unlayered, layered mirror wrapped + BOM-free, no catalog: literals, version untouched at 0.11.8 (bump lands in the release PR chore(release): react-ui 0.12.0, lang-core 0.2.6, cli 0.0.8 #630)
• openui-chat example smoke-built green under Next.js/Turbopack against the new export

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