Title screen: add Braille dithering modes

[flyingrobots]

Summary

Add optional dithering modes to the title-screen Braille renderer so ray-traced title scenes can trade harsh thresholded dots for more stable tone and smoother gradients.

Why

The current Braille title renderer maps traced or cached 2x4 subpixel samples directly into Unicode Braille dots. That is deterministic and fast, but it can look harsh around gradients, soft shadows, caustics, and low-frequency lighting. Dithering would let the renderer represent fractional luminance or coverage more gracefully without changing the terminal cell model.

Candidate modes

• Threshold baseline.
• Ordered Bayer 4x4 as the likely animated default because it is cheap, deterministic, and frame-stable.
• Atkinson as the first error-diffusion experiment.
• Floyd-Steinberg and Stucki later if the first modes prove useful.

Constraints

• Keep this inside the native Bijou/jedit renderer, not a browser canvas or DOM image pipeline.
• Dither over the full Braille subpixel plane, not independently per 2x4 cell, so error diffusion can cross cell boundaries.
• Preserve existing fg/bg RGB averaging, modifiers, ray stats, temporal cache, and adaptive trace budgets.
• Avoid shimmer during camera motion.

Design packet

A design doc should define the exact renderer contract, RED tests, and rollout plan before implementation.

[flyingrobots]

Design packet added in docs/design/0103-title-braille-dither-modes.md on branch title/perf-scene-facts.

This is intentionally parked behind the active Vim command-line completion work; implementation slices remain unchecked in the design doc.

[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.