Add minimum remaining-data checks for fixed-size binary reads

[I-WAS-FUTURE]

This change adds pre-read validation for operations where the required byte count is known in advance.

What changed:

• added EnsureCanRead(int byteCount)
• used it in InternalRead(Span<byte>)
• used it in ReadExactly(Span<byte>)
• used it in ReadString() after reading the encoded byte length

Why:

• fail earlier on seekable streams when there is not enough remaining data
• avoid starting partial reads for fixed-size values and length-prefixed strings
• improve behavior for truncated or malformed inputs without introducing hard size limits

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[I-WAS-FUTURE]

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