Add render-time color transforms and audio-reactive hooks

[avillagran]

Summary

Adds two opt-in embedding hooks for applications that drive TTFX as a library:

1. A render-time color pipeline for live, selective recoloring.
2. A per-effect audio hook, with Thunderstorm as the first reactive implementation.

Live color transforms

• Adds set_color_transform(Option<ColorTransform>) for current-thread render control.
• Applies transforms when cached CharacterVisuals are emitted, so palette changes are visible on the next frame without rebuilding an effect.
• Composes the selective transform before the existing audio brightness/hue transform.
• Handles both RGB and xterm colors while preserving indexed 38;5 / 48;5 output in xterm mode.
• Retains the original cached-symbol fast path when transforms are disabled and for colorless visuals.
• Uses Rc snapshotting and an owned pooled scratch buffer, so callbacks may replace themselves or construct nested visuals without RefCell borrow panics.

This hook is intentionally policy-free. The embedder decides which colors are semantic and which are theme accents.

Audio hook

Adds a default no-op method to Effect:

fn on_audio(
    &mut self,
    ctx: &mut EngineCtx,
    volume: f32,
    bass: f32,
    beat: bool,
) {}

The built-in CLI has no audio source and does not invoke it. Embedders call it immediately before next_frame.

Thunderstorm uses the hook to:

• increase strike probability with volume and detected beats;
• add up to ten simultaneous rays at high volume;
• scale branch count and strike progression speed;
• preserve the original non-audio execution path byte-for-byte.

The audio strike path reuses the ordinary flash/fade setup and uses VecDeque for queued rays.

Tests and verification

• cargo test --all-targets: pass.
• Registry smoke test discovers every Clap effect, builds all 37, invokes the audio hook, and verifies first-frame emission.
• High-volume Thunderstorm test runs 500 virtual-clock frames without stalling.
• Cached RGB visuals change across transform updates without effect rebuild.
• xterm transforms remain indexed SGR.
• Self-mutating callbacks and nested visual construction are covered.
• Non-audio Thunderstorm parity: 352 deterministic frames are byte-identical to origin/master (sha256 860c234f451a005ed5c953eeec41647a3a604d5040d7e2e163ae9e4c6cbaa964).
• Downstream Omarchy integration: all 35 non-local TTFX effects started and accepted live theme changes; 35/35 passed.

No behavior changes unless an embedder explicitly installs a transform or calls on_audio.