[MDA2AV]

The benchmark harness still called TryExtractFullHeaderROM and TryExtractFullHeaderValidated, which have been obsolete since #81. Building the solution printed CS0618 warnings for both calls.

• Both request benchmarks now call TryParseRequestHeader.
• They no longer call request.Clear() first. Since Clear the parse target before filling it #76 the parser clears its target itself, so the benchmark was clearing it twice.
• The multi-segment comment now describes Linearize only the header block of multi-segment input #86: the parser linearizes only the header block.

After this change, the solution builds with no CS0618 warnings. One run on an i9-14900K pinned to one P-core (.NET 10) stays within 4% of the README's tables:

	README	this run
Contiguous, ~95 B	120 ns	118 ns
Contiguous, 4 KB	750 ns	722 ns
Contiguous, 32 KB	5251 ns	5179 ns
Multi-segment, ~95 B	245 ns	237 ns
Multi-segment, 4 KB	1258 ns	1226 ns
Multi-segment, 32 KB	8695 ns	8338 ns
Chunked, 256 B	19 ns	20 ns
Chunked, 4 KB	115 ns	112 ns
Chunked, 32 KB	826 ns	806 ns