Surface variable modes in the Storybook toolbar

[nathanacurtis]

Problem

Split out of #609, which bundled a page with a host feature. This is the host feature.

Variable modes are emitted and never exercised. In the sealed run described on #574, the library defined Light and Dark and all 138 snapshots were taken in Light — half the library's surface went unrendered and unverified.

Scope

• Toolbar globals built from the emitted modes manifest, one control per collection.
• Selecting a mode sets the corresponding attribute on the document root, which the emitted custom-properties stylesheet already keys off.
• Works for component stories and foundations pages alike.

Notes

• This is host configuration, so it lands in the reference host documented by Document the complete reference Storybook host #607 and later becomes part of specs storybook init: scaffold the workspace host #610's template.
• It is what makes the color page (Color page, generated #609) and the other foundations pages pay off — without it a mode-aware token page can only show one mode.

Acceptance criteria

• A toolbar control per emitted collection, populated from the modes manifest rather than hardcoded.
• Switching a mode re-renders the current story against that mode.
• A library with a single mode produces no empty control.
• Document the complete reference Storybook host #607's documented host includes the configuration.

Case data

• Territory: cli
• Size: s
• Wave: 2