types/key: use AvailableBuffer for WriteRawWithoutAllocating (#19102)

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>

0 files changed, 0 insertions, 0 deletions