Use bufio.Writer.AvailableBuffer to write the 32-byte public key
directly into bufio's internal buffer as a single append+Write,
avoiding 32 separate WriteByte calls. Fall back to the existing
byte-at-a-time path when the buffer has insufficient space.
```
name old ns/op new ns/op speedup
NodeWriteRawWithoutAllocating-8 121 12.5 ~9.7x
(0 allocs/op in both)
```
Add BenchmarkNodeWriteRawWithoutAllocating and expand
TestNodeWriteRawWithoutAllocating to cover both fast (AvailableBuffer)
and slow (WriteByte fallback) paths with correctness and allocation
checks.
Updates tailscale/corp#38509
Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>
bb59942df2