Rail-to-tray responsive recipe + browser proof

[lbliii]

Saga: #329 · Epic: #334

Application chrome Five-Rock status: rail-to-tray recipes and browser proof are active backlog.

Goal

Add rail-to-tray recipe using existing sidebar, nav_tree, drawer, and tray; prove 320px stress, drawer fallback, badge overflow.

Tasks

• Write recipe doc: product rail → phone drawer fallback pattern.
• Browser gauntlet: 320px width, rail collapses to drawer, badges don't overflow, focus moves correctly on open/close.
• Showcase route or browser fixture demonstrating recipe.
• No new public macros — compose existing primitives.

Acceptance

• Browser gauntlet passes at 320, 768, 1280 widths.
• Recipe cited in docs/patterns/dense-navigation-recipes.md.

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Plan: docs/plans/PLAN-application-chrome-system.md Five-Rock table.