checkpt, try to parse packets only once

overlay/batch/batch.go

@@ -5,8 +5,15 @@ import "net/netip"
 type RxBatcher interface {
 	// Reserve creates a pkt to borrow
 	Reserve(sz int) []byte
-	// Commit borrows pkt. The caller must keep pkt valid until the next Flush
+	// Commit borrows pkt. The caller must keep pkt valid until the next Flush.
+	// Walks IP+L4 headers itself; prefer CommitInbound when the caller already
+	// has an RxParsed in hand from ParseInbound.
 	Commit(pkt []byte) error
+	// CommitInbound is Commit with a hint produced by ParseInbound, so the
+	// batcher can skip the IP+L4 re-parse. Borrowed slice contract is the
+	// same as Commit. Implementations that don't coalesce may delegate to
+	// Commit.
+	CommitInbound(pkt []byte, parsed *RxParsed) error
 	// Flush emits every queued packet in arrival order. Returns the
 	// first error observed; keeps draining so one bad packet doesn't hold up
 	// the rest. After Flush returns, borrowed payload slices may be recycled.

overlay/batch/passthrough.go

@@ -40,6 +40,13 @@ func (p *Passthrough) Commit(pkt []byte) error {
 	return nil
 }
 
+// CommitInbound ignores the hint — Passthrough never coalesces, so there's
+// no IP/L4 re-parse to skip. Present so Passthrough satisfies the RxBatcher
+// interface alongside MultiCoalescer.
+func (p *Passthrough) CommitInbound(pkt []byte, _ *RxParsed) error {
+	return p.Commit(pkt)
+}
+
 func (p *Passthrough) Flush() error {
 	var firstErr error
 	for _, s := range p.slots {