Eyevinn / mp4ff / 34780961367

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refactor(mp4): let DecodeStyp delegate to DecodeStypSR

DecodeStyp built the box itself instead of delegating the way DecodeFtyp
does, so the short-payload guard added in #581 had to be written twice
for styp and once for ftyp. Delegating removes the duplicate guard and
brings styp to the same dual-path shape as ftyp.

The two had already drifted: DecodeStypSR measured the payload as
int(hdr.Size)-hdr.Hdrlen where ftyp used hdr.payloadLen(). They are
equal today, but only one of them stays correct if a box ever carries an
extended size. Unify on payloadLen.

This also removes an observable inconsistency. For a styp that declares
a short payload and is also truncated, the reader path reported the
short payload while ftyp on the same input reported the truncation.
Both now report the truncation, which is what actually went wrong.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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