feat: four notes, seventh chords and a tesseract in the Notes & Chords lab

[benjamin-small]

Summary

• Four notes: single-note mode picks up to four; chord mode gains a Sevenths row (Dom7, Maj7, Min7, Half-dim7, Dim7) under the triads in the same radio group.
• Tesseract: with four notes the curve is drawn inside a tesseract (16 corners, 32 edges) turned through the fourth dimension — rotation in the x–w and z–w planes, perspective 4D→3D with a fit factor that keeps it inside the existing camera framing — drawn by the existing 3D pipeline. Fourth bar (soft blue #7FA8E8) in the bottom gutter.
• 4D controls: a 4D turn slider (0–359°) that follows the turn, and a Turn toggle; grabbing the slider holds the angle. The viz owns the angle (advanced in tick); a new Visualization::readout() → Engine::viz_readout() lets the page mirror it without per-frame config pushes (which would reset the cube's spin).
• Overlay: four swing labels (the bottom bar's hangs under its middle), the ratio readout moved to the strip's top-left corner, fourth legend swatch, copy and README updated.
• In pure ratios a C7 is 20 : 25 : 30 : 36 (the lab's 5-limit table makes the minor seventh 9/5), so it closes after 20 root swings.
• Spec: docs/superpowers/specs/2026-10-12-notes-tesseract-design.md; plan: docs/superpowers/plans/2026-10-12-notes-tesseract.md.

Validation

• cargo fmt --check, cargo clippy (only the 6 pre-existing Sierpinski warnings), cargo test 262 passed
• wasm (headless Chrome 155): 33 passed incl. notes_renders_one_to_four_notes
• vitest 414/414, svelte-check 0 errors
• Per-task reviews (8) + a whole-branch review; its findings (label overlap at the bottom-right corner on stages < ~517 px, stale thesis, untested Turn-hold, per-frame readout read, doc word) fixed and re-reviewed
• Browser (dev): #/notes?n=60,64,67,70&c=dom7 — tesseract turns, slider follows, no label overlaps at 1280×720 or 375×812, no console errors