In-memory head with WAL

Head connects chunkenc and WAL: label-to-ref resolution with striped
concurrent maps, active Gorilla chunk per series sealing at 120 samples,
and an Appender that buffers samples, writes WAL on commit, then applies
to head. OOO rejection checks both committed and batch state. WAL replay
on Open rebuilds the full in-memory state. Exported
ChunkAppender/ChunkIterator interfaces from chunkenc.

internal/chunkenc/xor.go

@@ -8,6 +8,18 @@ import (
 	"math/bits"
 )
 
+// ChunkAppender appends samples to a chunk.
+type ChunkAppender interface {
+	Append(t int64, v float64)
+}
+
+// ChunkIterator iterates over samples in a chunk.
+type ChunkIterator interface {
+	Next() bool
+	At() (int64, float64)
+	Err() error
+}
+
 type XORChunk struct {
 	b bstream
 }
@@ -20,7 +32,7 @@ func (c *XORChunk) NumSamples() int {
 	return int(binary.BigEndian.Uint16(c.b.bytes()))
 }
 
-func (c *XORChunk) Appender() (*xorAppender, error) {
+func (c *XORChunk) Appender() (ChunkAppender, error) {
 	if c.NumSamples() > 0 {
 		return nil, errors.New("chunkenc: appender on non-empty chunk")
 	}
@@ -134,7 +146,7 @@ func (a *xorAppender) writeVDelta(v float64) {
 
 // Iterator decodes the chunk. Snapshot semantics: it reads the byte slice
 // as it exists at creation; don't append concurrently.
-func (c *XORChunk) Iterator() *xorIterator {
+func (c *XORChunk) Iterator() ChunkIterator {
 	return &xorIterator{
 		br:    newBReader(c.b.bytes()[2:]),
 		total: uint16(c.NumSamples()),