Move media backends into one 'media' extra; base keeps the pure modules (#54)

[thorwhalen]

Closes #54. Under the commitment on reeleehq/reelee#541.

What changes

• Base install (pip install mixing) now declares only config2py, dol, xdol. The pure modules (srt, chapters, errors, util, egress, transcript, dubbing TTS/SRT) import and work with no media backend.
• One new extra, media: moviepy, opencv-contrib-python, scipy, pydub, burns, plus numpy, pillow and looks, which only the media modules import. The testing and dev extras include mixing[media], so CI still runs the whole suite.
• Clear failure without the extra: mixing.audio and mixing.video (and facade names resolving to them, e.g. mixing.Video, dub_video_from_srt) raise ImportError: ... requires the 'media' extra (missing: ...). Install it with pip install 'mixing[media]'. The check is util.require_media_extra, run at the top of both package __init__s via find_spec, so nothing heavy is imported to decide. util.require_package also names the extra for media libs imported lazily (e.g. scipy.signal).
• No public name removed or renamed. The lazy facade table is untouched.
• Docs (README, .claude/CLAUDE.md, the mixing skill) updated.

Tests

New mixing/tests/test_guard_base_install.py runs a fresh interpreter with the media libs hidden (sys.modules[name] = None): the pure modules import and load none of them; each media entry point raises the ImportError naming mixing[media]; the pyproject split is pinned. Full suite green locally with the extra installed.

In-fleet dependents

Found by rg over pyproject.toml/setup.cfg under the projects tree. One-line PRs change the dependency to mixing[media]; on a mixing without the extra pip only warns, so they can land before or after this release.

Dependent	Relies on mixing for media backends?	Action
muvid	yes (mixing.audio, mixing.video)	reeleehq/muvid#172
reelee	yes (assemble_audio_track, replace_audio, concatenate_videos)	reeleehq/reelee#646
braidio	yes (mixing.video)	reeleehq/braidio#103
paces	yes (mixing.audio)	reeleehq/paces#27
walkthru	yes (reelee and synth extras, mixing.audio)	reeleehq/walkthru#39
yb	yes (mixing.video.thumbnail)	issue reeleehq/yb#35 (outside the brief)
tituli, arioso, an	no mixing dependency declared	none
illustration	mentions mixing only in a comment	none

For syll-songs: a pinned mixing==0.0.55 is unaffected. An unpinned fresh install needs mixing[media].

Deploy: merging releases to PyPI. Anything that installs mixing unpinned in a fresh environment (reelee-iq deploy, syll-songs rebuilds) must carry mixing[media], or have the dependent PRs above merged first.

Review: small packaging change, tests only (per the review policy).

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