feat: color anti-aliasing for subcell-shape charsets

[tamirelazar]

Summary

Subcell-shape charsets (Braille, Quadrant, HalfBlock, Ascii) render shape at 2–4× resolution but carry a single foreground color per terminal cell, so thin diagonal veins staircase in color (visible even in --palette mono — it's brightness-domain, not a hue bug). This pre-blurs the per-cell color signal with a 3×3 kernel so color washes into a gradient while the glyph stays crisp (the shape path reads raw quadrant data, untouched).

What's included

• AaStrength { Off, Subtle, Strong } per-charset color-AA, stored as [AaStrength; 7] indexed by charset. Default: Braille = Strong, all others Off.
  • Strong = uniform 3×3 box blur; Subtle = weighted-center (0.5·self + 0.5·mean(neighbors)).
  • Blurs BOTH base brightness AND the temporal diff_norm accent field with one kernel.
• AA-eligible: Braille / Quadrant / HalfBlock / Ascii. Excluded: HalfBlockDual (already 2 colors/cell), Sculpted, Shade, Points.
• First-class parameter: round-trips undo/redo, reset-to-defaults, and saved configs (defensive on pre-feature configs).
• Surfaces: CLI --color-aa <off|subtle|strong> (auto: strong for braille, off otherwise), runtime " key (cycles active charset's AA), options-overlay "Color AA" row, --explain + README docs.
• Guarantee: when AA is Off or the charset is ineligible, output is byte-identical to before (guarded by a test using an ineligible charset at Strong).

Test plan

• cargo test --lib — 869 green (881 with --all-features)
• cargo test --test cli_integration / integration_app — green
• cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --check — clean
• Visual regression: 54 pass. Regenerated 3 AA-affected goldens (braille_mode, braille_brightness, etching). The lumen_preset golden is a pre-existing macOS-vs-Linux temporal divergence unrelated to this change (CI/Linux is source of truth).
• No allocations in blur hot loops (one Vec<f32> per blurred field per frame).

Notes

• The blur kernel is pure f32 add/mul/div (no transcendentals → deterministic cross-platform).
• Subtle kernel locked to weighted-center after a dev A/B (the directional variant was added then removed).