feat: add 'system' yt-dlp source (PR2 for #182)

[zibaone]

Context

Follow-up to #182 and #185 (merged). Adds system as a third value of ytdlp_source.

What it does

system uses a yt-dlp that is already importable by Kodi's Python. The addon downloads nothing — it only asks whether yt_dlp can be imported. Where the package lives is a packaging concern, resolved the same way deno_manager finds a system binary (shutil.which, no path setting).

Design decisions

• No path setting. From the addon's point of view the library is importable or it isn't. If system doesn't detect the library, that's a packaging problem — the package and its dependencies must be put where Kodi's Python can import them. The addon has no business knowing or configuring filesystem paths. This is a deliberate trade-off against the single-enum approach: a path setting would be a second configuration axis, but the alternative (silent failure) is worse.
• Fallback to stable. If system is chosen but no yt_dlp is importable, the addon reverts ytdlp_source to stable and shows a notification. This avoids a silent failure and ensures the addon continues to work.
• No auto default. On most platforms (LibreELEC, Windows, Android) nothing is importable, so silently skipping the download would be the wrong default. system is an explicit choice.

Changes

• core/ytdlp_manager.py: YTDLP_SOURCE_SYSTEM, is_managed_source(), resolve_system_ytdlp(), early returns in ensure_ytdlp_ready() and get_runtime_status(), guard in list_available_versions()
• core/runtime/actions.py: fallback to stable with notification when system has no importable library
• resources/settings.xml: third option system
• resources/language/.../strings.po: label #32072
• README.md: documents the importable-dependencies requirement
• tests/test_ytdlp_system_source.py: 9 new tests

Testing

• 294 tests pass (285 existing + 9 new)
• E2E validated: yt_dlp from Nix store detected, ready=True, runtime_path=None
• Kodi headless tested: notification + fallback confirmed

Notes

• No prototype changes, no new settings beyond the enum value
• Based on 413281c3 (firsttris's tidy-up of Add yt-dlp source selection: stable / nightly #185)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (413281c) to head (f0a4471).

Files with missing lines	Patch %	Lines
core/ytdlp_manager.py	95.65%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   72.79%   74.69%   +1.89%     
==========================================
  Files          14       14              
  Lines        2334     2363      +29     
==========================================
+ Hits         1699     1765      +66     
+ Misses        635      598      -37     

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[firsttris]

Thanks a lot, merged! 🎉 Really appreciate the quick follow-up and that you tested it on the NixOS box.

I pushed a small follow-up (75c5680) on top:

• system no longer rewrites the setting or downloads stable. A missing library is just reported (the importable check now uses the ensure_ytdlp_ready() result).
• With system, "Manage yt-dlp version" / "Update now" show the system version instead of the GitHub release list.
• The fallback tests now import actions fresh per test.

Would be great if you could give it a quick spin on your appliance with system selected. Thanks again for both PRs!

[zibaone]

Gave it the spin you asked for — both directions.

On the appliance (LibreELEC, Kodi 22.0-RC1, master incl. 75c5680), with system
selected and nothing importable, it reports and leaves everything alone — no
rewrite, no download:

08:02:50.748 warning  plugin.video.sendtokodi: yt-dlp source is 'system' but no yt_dlp package is importable by Kodi's Python
08:02:50.749 error    plugin.video.sendtokodi: yt-dlp import failed: No module named 'yt_dlp'

No "reverting to stable" anywhere in the logs.

On the NixOS side (importable case): kodi-21.3 with yt-dlp's site-packages pinned
on Kodi's PYTHONPATH — Using system yt-dlp 2026.08.19, DASH manifest built, aac
audio decoded, playback position advancing; nothing fetched, nothing installed.
The zero-config setup from #182 working as intended.

Also verified the nightly channel still works with both PRs merged — details in #185.

Thanks again for both merges and the follow-up fix — both are working well here.