fix(httpv2)!: frame Length counts the 9-octet header, so RFC 9113 frames are rejected #1121

Describe the bug
httpv2.py:367 writes Length as _make_http_length(...) + 9, and httpv2.py:298 reads it back the same way. RFC 9113 §4.1 excludes the 9-octet frame header from Length.

Separately, httpv2.py:637 checks PRIORITY against != 9, while sibling frames check payload + 9 (RST_STREAM checks 13). So pcapkit rejects even its own PRIORITY frame, whose Length is 14.

Reproduction

• An RFC-conformant PRIORITY frame (Length=5) raises ProtocolError.
• An RFC-conformant RST_STREAM frame (Length=4) raises ProtocolError.
• make_samples.py leaves out httpv2-frame/PRIORITY.

Expected behavior
Length excludes the frame header on both read and write, with the per-frame constants derived from that.

This is breaking: it changes the bytes make() produces, and which captures parse.

System information
main at bdba4fb9d, CPython 3.14.7, Linux.

Additional context
Found by the #1090–#1102 wave's audit and cross-reviews.