Renderer debt: physical Neon Dispersion features

[flyingrobots]

Follow-On Debt

Neon Dispersion is now implemented with the renderer primitives that exist today, but the literal cinematic prompt needs renderer capabilities that are not yet first-class.

Missing Capabilities

• Physical emissive tube lights that illuminate the scene as geometry.
• Spectral chromatic dispersion or wavelength splitting through refractive material.
• More faceted centerpiece primitives, such as torus, shattered crystal, or high-poly icosahedron meshes.
• Roughness maps or blurred floor reflections for wet/scuffed metal.
• Depth of field and bokeh for foreground/background floating spheres.
• Finite, one-sided, or bounded room geometry. Current authored walls are infinite planes, which can occlude the whole title scene when the camera orbits outside the intended room.

Acceptance Criteria

• Scene JSON can express these capabilities without hardcoding Neon Dispersion in renderer internals.
• Tests prove each capability through inspectable scene facts or deterministic render output.
• scenes/neon-dispersion.jedit-scene can be upgraded to use the new fields while preserving fallback behavior for older scene assets.
• A future room/backdrop implementation cannot collapse a camera orbit sample to a flat wall/background.

Follow-on from #101 / #102.

[flyingrobots]

The title screen no longer renders on the startup path. As of 14d17b5 and 910373f on #304, jedit opens on an empty viewer, createInitialModelSnapshot loads no meshes and preloads no scene, and the ray-traced backdrop is reachable only by explicit opt-in (the scene picker, the title number keys, or an injected renderer). Two further fixes on #321 closed paths that were switching it on by accident — arrow keys and pointer movement both ran through activateLegacyTitleBackdrop.

This work is therefore no longer on any path a normal launch touches. It is deferred behind the Easter-egg opt-in, not obsolete.

Leaving it open and unlabelled rather than closing: the capability it describes is still real work if and when the ray-traced title returns as a deliberate feature. It just carries no urgency now, and should not be counted against the editor's day-to-day quality.