Scroll-reveal: add --stagger, -all (children), blur and parallax variants

[jackgranatowski]

Problem

.sf-entrance--* (fade/fade-up/-down/-left/-right/scale-up) ships and is scroll-driven via animation-timeline: view(). It's missing the higher-value scroll-reveal ergonomics: staggering a group of children, applying to all direct children at once, and blur/parallax flavours.

Scope

• --stagger progressive child delay (CSS sibling-index() is within our browser floor).
• -all variant that applies the effect to direct children of a container.
• New effect flavours: --blur (blur→sharp) and --parallax.
• All gated on prefers-reduced-motion; graceful fallback (immediate show) where view() is unsupported, as today.

Acceptance

• .sf-entrance-all--* + --stagger working without JS
• blur + parallax variants
• Reduced-motion safe + documented