Content-grid: add feature-max and full-safe breakout zones

[jackgranatowski]

Problem

.sf-content-grid currently exposes three zones: content / breakout / full-bleed. Editorial and marketing layouts often want an intermediate "feature-max" tier (wider than breakout, narrower than full) and a "full-safe" zone (full width but keeping the gutter). Both are missing.

Scope

• Add a second breakout tier (feature-max) between breakout and full, plus a full-safe column that spans full width minus gutter.
• Extend the grid-template named lines + corresponding child classes (.sf-feature-max, .sf-full-safe) and --sf-*-width tokens.

Acceptance

• Two breakout tiers + full-safe available and nestable
• Documented