derp: use AvailableBuffer for WriteFrameHeader, consolidate tests (#19101)

Use bufio.Writer.AvailableBuffer to write the frame header directly
into bufio's internal buffer as a single append+Write, avoiding 5
separate WriteByte calls. Fall back to the existing writeUint32
byte-at-a-time path when the buffer has insufficient space.

```
name                  old ns/op  new ns/op  speedup
WriteFrameHeader-8    18.8       7.8        ~2.4x
(0 allocs/op in both)
```

Add TestWriteFrameHeader with correctness
checks, allocation assertions, and coverage of both fast and slow
write paths. Move BenchmarkReadFrameHeader from client_test.go to
derp_test.go alongside BenchmarkWriteFrameHeader, co-located with
the functions under test.

Updates tailscale/corp#38509

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

0 files changed, 0 insertions, 0 deletions