Color-by-temporal-change + colorize-last invariant (levers 3 & 9)

[tamirelazar]

Implements the temporal-color art-direction lever (#29) on top of the colorize-last invariant fix (#28). Closes #28, closes #29.

Global default OFF — a bare run, the Organic default, and all existing presets/saved-configs are byte-identical to before. The feature only activates when a user passes --temporal-color > 0 (or selects a future showcase preset).

What changed

Lever 9 — colorize-last invariant

• Extracted palette::colorize_subpixel: a single shared colorize pass now used by every output (TUI frame buffer, GIF, WebM, headless print, frame capture, and PNG save).
• Fixed interactive PNG save, which previously mapped colors with mapping = None and so skipped intensity-mapping (and would have skipped all new color levers). Saved stills now match the live look.
• Added a module-level invariant doc and a PNG/render colorize-parity guard test.

Lever 3 — color-by-temporal-change

• Sim maintains an EMA temporal_lag buffer and emits a raw signed diff = trail − lag (positive = growing front, ~0 = settled vein, negative = decaying tail). Allocated only when temporal is enabled.
• The signed diff is average-pooled through downsampling into AuxCell.signed_diff (sign preserved), normalized by the white-point in the colorizer, then modulated:
  • hue mode — rotate the base OKLch hue by tanh(k·diff_norm)·max_shift (front/tail rotate opposite directions).
  • accent mode — blend toward the palette's hot end in OKLch (Bleuje-style; most dramatic).
• Exposed as --temporal-color <0..1>, --temporal-lag <frames>, --temporal-mode <hue|accent>, plus RuntimeState fields and saved-config persistence (#[serde(default)] so old TOML loads unchanged).
• Applies across TUI, GIF, WebM, frame-capture, print, and saved PNG.

Known limitation

Loading a saved config with temporal_color > 0 does not re-enable the sim computation live (temporal stays off until restart with the flag). This fails safe (base colors, never wrong) and mirrors the existing trail-age/delta/gradient CLI-only-at-startup behavior; the proper fix belongs with the config live-apply / cache-sync refactor, not this change.

Test plan

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test (774 lib + integration + doc)
• cargo test --test visual_regression — 47 pass; all 46 pre-existing goldens unchanged, 1 new temporal-on golden added
• Default-off invariant traced end-to-end (buffers None → diff 0 → colorizer returns the legacy mapping); parity + seam tests lock it
• Manual: -S --preset organic --temporal-color 0.8 --temporal-mode accent colors the growing front; --temporal-color 0 is unchanged
• Manual: press g to save a PNG (with a non-default --intensity-mapping) and confirm it matches the screen
• Manual: GIF export with vs without --temporal-color differs