Fix dref url self_contained flag bit position

[iameli]

Noticed that this library was setting 0x000002 on the url box and couldn't figure out why; Claude figured out it was a bug. Neat.

From Claude:

The self_contained flag in the url box should be bit 0 (0x000001) per ISOBMFF §8.7.2, but was defined at bit index 1, producing flags=0x000002. This could cause strict parsers to interpret the data reference as non-self-contained.

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ℹ️ Recent review info

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Configuration used: Organization UI

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Run ID: 228ede60-880a-42e4-8333-8a508e45abb6

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Reviewing files that changed from the base of the PR and between 3073b3e and 212b44c.

📒 Files selected for processing (1)

• src/moov/trak/mdia/minf/dinf/dref/url.rs

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Walkthrough

The change modifies the default behavior of the Url extension in the ISOBMFF format handling. Specifically, the self_contained flag default is changed from 1 to 0 in the public Url extension declaration. Additionally, during encoding, the UrlExt initialization is updated to include ISOBMFF conformance documentation and explicitly sets self_contained to true, replacing a previous placeholder comment. These modifications affect how the URL extension interprets media data containment within the file format.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing the bit position of the self_contained flag in the dref url box from bit 1 to bit 0, which is the core purpose of this PR.
Description check	✅ Passed	The description clearly explains the bug being fixed: the self_contained flag was at the wrong bit position (bit 1 instead of bit 0 per ISOBMFF), which is directly related to the changeset modifications.

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[bradh]

Fix looks good. Needs a test case.