Fix DASH progress bar and check that the remux succeeded

[sjbrownrigg]

As offered in my comment on #1024 — here are the two fixes, on top of your dev.

Worth saying: the hard part was yours. Working out that Tidal serves HI_RES_LOSSLESS as an MPEG-DASH manifest, and that the old code was silently falling back to a lower quality when the JSON parse failed, is the reason this works at all. These are just two rough edges I hit while running it.

1. Progress bar shows every track as instantly complete

TidalDASHDownloadable inherits size(), which HEADs self.url — for a DASH track that's only the init segment, a few hundred bytes against a track of tens of MB. So the progress total is tiny and the first chunk takes it past 100%:

Track 5 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 3.9 MB/s • 0:00:00
Track 6 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 5.5 MB/s • 0:00:00

Now sums the init segment plus every media segment.

One detail that cost me a debugging cycle, in case it looks over-engineered: issuing one HEAD per segment concurrently got roughly half of them refused, and treating a refusal as zero bytes reported 17.0 MB for a track that downloaded 34.6 MB — so the bar was then wrong in the other direction. Hence the semaphore, and estimating any stragglers from the average rather than dropping them.

Verified by counting bytes handed to the progress callback: 34.59 MB reported against 34.59 MB delivered.

2. A failed remux is treated as a successful download

If ffmpeg fails, the temp file is removed, no output is written, and _download returns normally — so the caller counts the track as downloaded with nothing on disk. On current dev that also records it in the downloads database, so it's skipped on every future run and the track is quietly missing for good.

Now checks the exit status and that the output exists, and raises NonStreamableError otherwise, with ffmpeg's stderr in the message.

Notes

• No new dependencies; asyncio and os were already imported at module level, so I dropped the two function-local imports.
• Test suite is unchanged at 59 passed / 1 failed, and that failure (test_meta.py::test_album_metadata_qobuz, a genre assertion) also fails on an unmodified nathom/streamrip dev, so it's not from this.
• I left README.md alone. My only thought there is that the "Changes from upstream / 🚨 End Fork Changes 🚨" section reads as fork documentation, so it may attract review comments if this goes upstream — entirely your call.

Tested against a Tidal "Max" subscription: q=2 returns AAC 320kbps in an m4a, q=4 with your changes returns 24-bit FLAC (96kHz on one album, 48kHz on another), decoding cleanly.

Happy to change anything here, or split it into two if you'd rather take them separately.