migrate scale builder to scored exercise

Created by: jedmund

Rebuilds Scale Builder on top of the scored-exercise architecture (ExerciseState + ScoringOverlay), matching the Interval exercises. It now records per-lesson attempts, restores prior state, and presents its UI inside a glass card.

On top of the migration, this PR parameterizes the exercise the same way the Interval exercises are configurable, fixes a KeyboardContainer overlay anchoring issue, and polishes the in-card feedback.

Changes

Migration

• Move Scale Builder onto ExerciseState / ScoringOverlay / PromptResult, with per-prompt review on completion.
• New glass card with step-arc overlay docked above the keyboard.

Keyboard overlays (sidebar + arcs)

• KeyboardContainer now anchors docked overlays via an alignment guide so step arcs and degree labels sit above the keys without callers compensating with a negative offset. The playground's -overlayHeight shim is gone.
• Playground configurator sidebar is constrained above the docked keyboard using a DockedKeyboardHeight preference key.

Parameterization

• Configurator gains a Scale Builder section (replacing the keyboard-tool controls that leaked in during migration): root multi-select (chromatic), mode multi-select (all 7 classical modes), total questions stepper, progressive-difficulty toggle, show-pattern-hint toggle.
• Progressive mode unlocks roots along the circle of fifths and modes in difficulty order (one new entry per correct answer).
• Exercise samples a (root, mode) pair each question and prefers unseen pairs; card title reads Build a scale from <Root> <Mode>.
• ScaleBuilderConfig.Config gains optional totalQuestions, progressiveRoots, progressionThreshold, showPatternHint, modes, and progressiveModes — lesson JSON (e.g. 2-ex4.json) still loads unchanged.
• Converts unicode ♭/♯ to ASCII b/# before calling Pitch(ascii:), which silently partial-parses unicode and produced the wrong pitch (e.g. A♭4 → A4).

UX polish

• Before the first note the card shows "To start, play Note below", crossfading to the pattern pills once the root is played.
• Unsolved pattern pills use primary/10% (matching the progress dots) instead of near-black.
• Wrong key presses trigger a short horizontal shake on the pattern hint.
• Root key is drawn with the accent (orange) treatment; the most recent correct press gets the highlight (blue) treatment so both read at a glance.

Test plan

• Standalone: home → Scale Builder card, build the scale, retry, inspect step arc placement
• Lesson: Music Theory lesson 1 exercise 2-ex4 → context markdown renders, card uses interactiveBackground, attempt is recorded and restores on re-open
• Playground sidebar: Scale Builder section shows only exercise parameters (no keyboard size / octave / overlays / highlights)
• Fixed mode: all 12 roots + Major, 8 questions, sampling avoids immediate repeats and seen pairs
• Progressive mode: starts with 3 roots unlocked; each correct answer unlocks a new root and mode until pool is exhausted
• Pattern hint: "To start, play X below" shows first, fades to pills after root; wrong key shakes the pills
• Root vs. last-correct: root key is orange, last correct key is blue, both persist during the scale
• Accidentals: pick a root like B♭ or F♯ — keyboard highlights the correct key and the expected sequence starts on that pitch
• WH pattern: pick a config using the WH step token, confirm the sequence still validates
• Free play keyboard: enable Step Arcs + Degree Labels, confirm they sit above the keys