Use conservative byte-size estimation for fixed byte-sized segments

[Bolajiadesina]

Context

• Related to #2639: “Create byte sized segments” sometimes produces segments larger than the specified max size.
• This PR is a small step toward stricter size enforcement by using a more conservative duration estimation.

Effect

This is still an approximation, but:

• We now aim for slightly smaller segments than the user’s limit, accounting for overhead.
• This should significantly reduce cases where segments end up noticeably above the requested byte size.

It does not yet implement per‑frame back‑off.