feat(config): single-scene webcam/blank default to infinite duration

[kfox]

Problem

Running a single webcam scene produced a periodic (~every 30 s) audio pop, a screen glitch, and a repeated burst of Mic Audio INFO lines. Root cause: a webcam scene has no natural length, so it fell back to the base Scene.duration_s = 30.0, and a single-scene playlist loops the one scene via a full teardown() + setup(). Each loop stopped/restarted the audio streamer (the pop), reopened the mic + rebuilt the DSP chain (the reprinted lines), and re-engaged the display mode (the glitch). YouTube looked clean because VideoScene runs video-driven to math.inf and never re-inits.

The 30 s default suited a convention demo, but the common webcam use case is "leave it running."

Change

• Single-scene webcam/blank default to infinite. build_scene sets duration_s = math.inf for webcam/blank when they're the sole configured scene (interleaving is skipped for a 1-scene playlist, so scene count is the whole story). A multi-scene playlist keeps the finite 30 s default, because an infinite live scene never becomes is_done and would wedge the rotation.
• duration_s = 0 is a universal "run forever" sentinel (mapped to math.inf for any non-video type). Negative durations are rejected at load. No schema/serializer change needed — SceneCfg.duration_s round-trips as a plain float; math.inf only lives on the built Scene.
• Startup log formats inf as duration=unbounded (was video-driven, now accurate for the non-video case).
• Docs (example TOML, architecture.md), field help, and regenerated JSON schema updated in the same change set.

Testing

• make test (1902 tests), make typecheck (mypy strict + pyright), make lint, make schema — all green. New tests cover the single-scene infinite default, the multi-scene 30 s default, the 0 sentinel, positive durations, and the negative-duration rejection.
• Hardware (U64): a single webcam scene ran ~77 s — past both old 30 s and 60 s loop points — with exactly one scene setup / mic init / DSP-active line and zero mid-run re-setup bursts.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.56%. Comparing base (5171975) to head (733c36d).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
c64cast/config.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #71   +/-   ##
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  Coverage   79.55%   79.56%           
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  Files          76       76           
  Lines       15420    15426    +6     
  Branches     2318     2321    +3     
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+ Hits        12268    12274    +6     
  Misses       2622     2622           
  Partials      530      530           

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