feat(audio): record several playback devices as separate clip tracks

[vxnsin]

What does this PR do?

Adds a "Record" choice to Settings > Audio: one device (as before), each program as its own track, or several playback devices as separate named tracks in the clip.

Why?

With a hardware mixer (for example a RODECaster with Chat, Game and Music outputs) only one playback device could be recorded, and per-program tracks were tied to the "separate system and microphone tracks" switch. Several devices as their own tracks lets the clip editor balance each source on its own.

Changes

• Settings > Audio: new "Record" selector. For "Several devices" you pick a playback device, name the track and add it (up to 4). The playback device picker is only shown for "One device".
• Engine: each added device is captured with WASAPI loopback and written as its own named track plus a manifest entry. Idle devices are padded with silence so a device that goes quiet stays on the timeline. The combined Default Mix is left out in this mode.
• The muxer no longer requires the Default Mix to be the first track. An unavailable system endpoint used to fail the whole save (Error 015); now only that track is dropped.
• Two new engine launch arguments: 26 (device list as name;endpoint|...) and 27 (per-program flag, defaults to the separated-mode value). Names are limited to the character set the audio manifest accepts.
• Clip track limit and manifest reader grow from 10 to 14 sources.
• Saved choice (audio_source_mode) wins; older settings derive it from the per-program switch / separated mode, so existing setups behave as before.

Testing

• Ran QT_QPA_PLATFORM=offscreen pytest tests/ — all pass except test_sftp_provider (upload bundle not present in a source checkout)
• Tested manually on Windows
• Native suite (FTHRclips_tests): 22624 checks pass, including new parser/limit tests.
• Saved clips in all three modes on an AMD GPU: "several devices" produced the expected named tracks (a device with no audio is skipped with a log line), "each program" and "one device" kept their previous tracks.

Notes

• The AMD fix for odd window sizes lives in its own PR and is not part of this branch.
• A device that cannot be opened (for example one held exclusively by another app) is logged and skipped; the clip is still saved with the remaining tracks.

Screenshots

One device:

Each program as its own track:

Several devices as separate tracks: