SPEC.md: rewrite Pivot page description to cover named layouts, depth
control, selection mode, inspector filtering, and layout persistence.
docs/perspective-pivot.md: new file documenting all discovered Perspective
v4.4.0 APIs — viewer/plugin/view methods, selection modes, set_depth
mechanism, perspective-click event shape, full state save/restore pattern,
and common pitfalls.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
