derp,types,util: use bufio Peek+Discard for allocation-free fast reads (#19067)

Replace byte-at-a-time ReadByte loops with Peek+Discard in the DERP
read path. Peek returns a slice into bufio's internal buffer without
allocating, and Discard advances the read pointer without copying.

Introduce util/bufiox with a BufferedReader interface and ReadFull
helper that uses Peek+copy+Discard as an allocation-free alternative
to io.ReadFull.

  - derp.ReadFrameHeader: replace 5× ReadByte with Peek(5)+Discard(5),
    reading the frame type and length directly from the peeked slice.
    Remove now-unused readUint32 helper.

    name                  old ns/op  new ns/op  speedup
    ReadFrameHeader-8     24.2       12.4       ~2x
    (0 allocs/op in both)

  - key.NodePublic.ReadRawWithoutAllocating: replace 32× ReadByte with
    bufiox.ReadFull. Addresses the "Dear future" comment about switching
    away from byte-at-a-time reads once a non-escaping alternative exists.

    name                              old ns/op  new ns/op  speedup
    NodeReadRawWithoutAllocating-8    140        43.6       ~3.2x
    (0 allocs/op in both)

  - derpserver.handleFramePing: replace io.ReadFull with bufiox.ReadFull.

Updates tailscale/corp#38509

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

derp/derpserver/derpserver.go

@@ -52,6 +52,7 @@ import (
 	"tailscale.com/tstime/rate"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/bufiox"
 	"tailscale.com/util/ctxkey"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
@@ -1088,10 +1089,10 @@ func (c *sclient) handleFramePing(ft derp.FrameType, fl uint32) error {
 		// space for future extensibility, but not too much.
 		return fmt.Errorf("ping body too large: %v", fl)
 	}
-	_, err := io.ReadFull(c.br, m[:])
-	if err != nil {
+	if _, err := bufiox.ReadFull(c.br, m[:]); err != nil {
 		return err
 	}
+	var err error
 	if extra := int64(fl) - int64(len(m)); extra > 0 {
 		_, err = io.CopyN(io.Discard, c.br, extra)
 	}