Diffusion/decay art knobs: afterglow, decay-gamma, separable Gaussian, diffuse-weight

[tamirelazar]

Implements lever 7 (#30): four art-direction knobs on the diffusion/decay stage. All global-OFF / back-compat-neutral by default — a bare run and every existing preset are byte-identical to before; the 47 visual-regression goldens are unchanged.

Knobs (CLI + saved-config + per-preset default)

• --afterglow <0..1> + --afterglow-rate <α>: EMA afterglow buffer (afterglow_lag) folded into the blended trail before downsampling, so the glow participates in white-point normalization. Luminous lingering tails. Default 0.0 (off).
• --decay-gamma <0.25..2.0>: value-dependent decay new = cur·(1 − (1−factor)·(cur/peak)^(1−γ)). γ<1 lengthens faint tails without blooming cores; γ=1 delegates to the existing decay (byte-identical). Default 1.0.
• --diffusion-sigma range widened 0.5–2.0 → 0.5–4.0, backed by a separable Gaussian (two 1D passes, radius ceil(3σ)) routed only when σ>2.0 — the existing 5×5/3×3 paths are untouched.
• --diffuse-weight <0..1>: Lague blend new = old·(1−w) + blur·w. w=1.0 is a full-weight fast path, byte-identical to today. Default 1.0.

Design notes

• Afterglow is a render-time fold into the transient blended buffer; it never mutates the trail map (sim dynamics unchanged). Wired at all five render entry points (interactive runner + print/capture/gif/webm export modes).
• New knobs live in SimConfig; saved-config fields are Option<T> with #[serde(default)] so old TOML loads unchanged. The four new validation rules are enforced in SimConfig::validate.
• No per-frame heap allocation on the default path: the afterglow EMA buffer is allocated once on enable; the diffuse-weight snapshot buffer is reused and only taken when w<1.0.

Verification

• 789 lib tests + 47 goldens (unchanged) + clippy -D warnings clean + fmt clean.
• Empirical same-seed check: every knob at its off/neutral value is byte-identical to baseline; on-values produce different frames with sensible direction (afterglow +46%, decay-gamma 0.5 +318%, wider sigma +27% lit cells).
• Benches added for separable Gaussian, decay-gamma, afterglow EMA.

Spec: docs art-direction levers design §4–§8 (lever 7).