feat(songLyrics v2): add explicit line end timing

[ranokay]

Summary

line.start tells a client when a lyric line begins, but it cannot describe when the final line ends or distinguish a deliberate gap, an overlap, and an instantaneous marker. This adds an optional line.end value so enhanced lyrics clients can represent those timings without inferring them from the next line.

Compatibility

• end uses the same track-relative millisecond timeline as start; the structured-lyrics offset applies to both values.
• It requires start, must be greater than or equal to it, and may equal it for an instantaneous marker. Gaps and overlaps remain valid.
• An omitted end means that the end is unknown. It does not imply the next line's start.
• The field is available only in enhanced=true responses. Unsynced lyrics omit both timing values, while default responses and the advertised [1, 2] versions remain unchanged.

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[Tolriq]

Its omission means the end is unknown; clients **must not** assume the line lasts until the next line starts.

If there's no end clients needs to assume things, so the wording is probably too strong.

[ranokay]

You're right. I was trying to distinguish an exact, source-provided end from an end inferred by the client, but yeah, must not is too restrictive.
I'll change the wording to:
Its omission means the exact end is unknown. Clients may infer a fallback end, for example from the next line's start, but should not treat it as exact source timing.

Thanks for pointing that out.

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