feat(pedagogy): quadrics audit + refinements (§10.6)

[bradleypmartin]

Sub-issue of #233.

Per-scene pedagogical coverage audit for quadrics against APPM 2350 §10.6, and the resulting capability additions. Sub-epic — audit-phase findings get broken out into real sub-sub-issues at plan-doc time. Pedagogically close per initial read; primarily polish + small additions rather than substantive new content.

Starter audit findings (from initial planning pass)

• Cross-section refinement. Today's cross-section presentation likely warrants polish; audit specifics at plan-doc time.
• Surface transparency when cross-sections active. When a cross-section is displayed, the quadric surface should be more transparent so the entire conic section reads, not just the visible arc above the cutting plane.
• Toggleable conic-section equation readout. For the current cross-section constant, what equation does that slice satisfy? Surface as a toggleable readout so the visitor sees the conic-section equation alongside the visual.
• Additional gaps surfaced during full audit at plan-doc time.

Open questions (defer to plan doc)

• Cross-section equation readout: plinth-mounted (per E1.4 rule for "knobs and dials") or math-object overlay (per E1.4 rule for "direct-manipulate-the-math-object")? Lean plinth-mounted — the readout reports state, doesn't enable direct manipulation.
• Does the conic-equation logic want a shared EquationReadout scaffold primitive (extract-on-second-consumer per polish: cross-cluster + audience-prep sweep (v0.9 epic) #188), or stay quadrics-local until a second scene wants it?

Composition

Paired with the quadrics narrative (#228). Plan together — items here surface during narrative drafting and vice versa; consider bundling into paired PRs.