Fractional width utilities (.sf-width-*)

[jackgranatowski]

Problem

We expose named container widths but no fractional content-width helpers (e.g. constrain an element to ~30/50/70% of content width with auto-centering). This is a common quick-constraint on marketing layouts.

Scope

• .sf-width-10 … .sf-width-90 (or a sensible subset) = max-inline-size: calc(var(--sf-content-width) * fraction) + inline-size: 100% + centering; plus full/auto/fit/min/max-content.
• Optional --sf-width-* tokens.
• Ship in the opt-in utilities layer; coordinate with the curated-utilities effort (Utilities: curated core set #248).

Acceptance

• Fractional width utilities emitted (opt-in), logical properties
• Documented