fix: remove duplicate representation entries for absolute BaseURLs

[cpruijsen]

Description

buildBaseUrls dedupes the cartesian product by exact object identity (first copy wins). When an MPD has more than one parent BaseURL (CDN / content steering) and a child BaseURL is absolute, that child is no longer emitted once per parent, so representationInfo does not contain duplicate playlist entries. Fixes #175.

resolveUrl returns a scheme-bearing URL unchanged, so the product is N identical objects. Parent serviceLocation is not copied in that case, so the copies are exact duplicates.

Relative parent×child combinations are unchanged. Content-steering copies that differ by serviceLocation (or any other attribute) are kept.

The TODO that used to sit on inheritAttributes described these duplicates. On PR #17, mjneil described the same cartesian product: the subtitle representation is duplicated because the absolute child is resolved against each parent BaseURL; the fix belongs in base URL creation.

Decision

Drop exact-duplicate resolved BaseURL objects in buildBaseUrls.

Alternative: skip the product only when the child URL did not resolve (resolvedUrl === childUrl), or unique the final representationInfo list.

The product is built in buildBaseUrls, and exact-object identity is the smallest key that still keeps distinct serviceLocation copies. Open to uniquing later or special-casing only absolute URLs.

Identity is JSON.stringify of the resolved object. Objects are always built as merge(parseAttributes(el), { baseUrl }) plus an optional serviceLocation assignment, so key order is stable for cartesian clones. Two identical <BaseURL> nodes written in the MPD (not a cartesian artifact) also collapse; copies that differ by any attribute do not.

Example

<MPD>
  <BaseURL serviceLocation="alpha">https://cdn1.example.com/</BaseURL>
  <BaseURL serviceLocation="beta">https://cdn2.example.com/</BaseURL>
  ...
  <Representation id="en">
    <BaseURL>https://example.com/en.vtt</BaseURL>
  </Representation>
</MPD>

representationInfo for en is one entry (https://example.com/en.vtt), not two. Video representations still have one playlist per CDN.