Closes #17139

Changes

• Wraps all CSS generated by generateImageStylesCSS() in @layer astro.images { … }. Per the CSS cascade spec, unlayered styles always beat layered styles regardless of specificity — so the previous unlayered :where([data-astro-image=constrained]){max-width:100%} was silently overriding any user max-width defined inside a @layer, even though :where() has zero specificity. Putting these defaults into a named layer restores the original intent of :where(): Astro's image styles should always be easy for users to override.
• The layer name astro.images uses a dotted namespace to avoid collisions with user-defined layer names while remaining valid for CSS processors (e.g. lightningcss).
• Note for users: to guarantee astro.images sits below your own layers, declare the order explicitly: @layer astro.images, layout, page, components;. Without an explicit order declaration, the first-seen layer wins — which may or may not be astro.images depending on stylesheet load order.

Testing

• Updated assertions in packages/astro/test/units/assets/utils.test.ts to expect the @layer astro.images { … } wrapper around the generated CSS output.

Docs

• No docs update needed. image.responsiveStyles is already documented as producing overridable defaults; this fix makes that promise actually true when users write CSS layers.