feat(core-ui): add CookieConsentBanner headless component

Implements the EU-compliant cookie consent banner with:
- modal + banner variants, CNIL equal-prominence Reject/Accept buttons
- granular category toggles (essential non-toggleable)
- __consent_state cookie management (SameSite=Lax, Secure, 1-yr, _v:1)
- render-prop overrides: renderHeader, renderCategoryRow, renderActions
- useConsent() integration when ConsentProvider is present
- CookieConsentBannerLoader SSR-safe wrapper
- RTL behavioral tests: Reject All, Save Selected, ESC=Reject, focus-trap
- Storybook stories for modal, banner, render-prop, and a11y tab demo
- jsdom configured with HTTPS origin for Secure cookie testing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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