Dead code left over from the /create panel consolidation

[mehdibha]

Orphaned after the config.tsx / lab.tsx / layouts.tsx / macros.tsx consolidation in #511 — all confirmed to have no remaining importers:

• panel/schema.tsx: ALL_CONTROLS and sectionOfControl exports (the latter's docstring describes a command-palette consumer that no longer exists); SeedField no longer needs to be exported.
• create/colors/index.tsx: ColorsSummary and ColorsConfig — their only consumer was the deleted customizer-panel.tsx.
• panel/primitives.tsx: the hint prop on Segmented is a dead option, and as written would replace the visible label for screen readers via aria-label rather than supplement it.
• panel/schema.tsx: the const c = (control: Control): Control => control identity helper is vestigial now that SECTIONS is annotated Section[].
• create/export/export-dialog.tsx: stale comment referencing "the panel lab's footer button".

Also: 'shape' status hardcodes designSystem.density !== 'default' instead of comparing against DEFAULTS.density — equivalent today, but the only chapter restating its default inline.

[mehdibha]

All named symbols are gone with the old modules/create module (#664, #677); the 'shape' density hardcode was replaced by normalization in axes/space.ts.