fix(examples): audio-mixer-demo's processors take a config class instead of keyword parameters

[tato123]

What & why

examples/audio-mixer-demo's processors still take their configuration as keyword parameters on __init__. Since v0.21.0 (#2226), @processor refuses that signature at decoration, so importing the processor module raises and the example no longer starts.

This is the consumer backlog the agent-readable-processor-catalog change owes at ship (docs/plan/changes/archive/2026-09-13-agent-readable-processor-catalog.md). Per docs/plan/ARCHITECTURE.md §Consumers, a converted consumer's breakage is filed at that consumer and never blocks the engine change.

Design

Each processor below takes one config parameter annotated with a config class, and reads its values off that object. The config class is a dataclass whose fields are today's keyword parameters with their defaults, or a TypedDict. rt.add(..., config={...}) in the app stays as it is: the helper builds the config class from the dict. examples/camera-virtual-camera/processors/shader_effect.py (ShaderEffectConfig) is the in-tree model.

• examples/audio-mixer-demo/processors/chord_mixer.py — ChordMixer(voice_gain)
• examples/audio-mixer-demo/processors/tone_source.py — `ToneSource(frequency_hz, sample_rate, channels, block_size, amplitude)"

Done means

• Every processor above declares def __init__(self, config: <ConfigClass>), and no @processor class in the example takes a keyword parameter.
• The example starts against the published wheel and reaches a running graph.

Validation shape

A fresh uv sync in the example, then run it as its README says. The processor modules import without a decoration refusal and the graph reaches Running.

Needs the physical rig?

audio

Non-derivable notes

Examples pin the published wheel, so validation needs a release carrying #2226 and #2228 (v0.22.0 or later). A local maturin develop is undone by uv sync.