fix(nwsync): hash through a section reader, pin the encoder claim (#76)

Review follow-ups. checkArtifactKey drained the *os.File to EOF, so the
file offset was left at the end; correct only because everything after it
uses ReadAt. Hash a section reader instead, so no later sequential read
can silently see nothing.

The zstd concurrency comment claimed byte-identical output but nothing
asserted it, so add the test. erf.Read now says outright that Data must
not be mutated, since payloads alias one buffer.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/nwsync/emit.go

@@ -88,7 +88,9 @@ func Emit(options EmitOptions) (EmitResult, error) {
 	if err != nil {
 		return EmitResult{}, fmt.Errorf("read artifact: %w", err)
 	}
-	if err := checkArtifactKey(options.ArtifactKey, artifact); err != nil {
+	// A section reader, not the file itself: hashing must not move the file
+	// offset out from under everything that reads the artifact afterwards.
+	if err := checkArtifactKey(options.ArtifactKey, io.NewSectionReader(artifact, 0, info.Size())); err != nil {
 		return EmitResult{}, err
 	}
 	name := options.As

internal/nwsync/memory_test.go

@@ -1,7 +1,9 @@
 package nwsync
 
 import (
+	"bytes"
 	"fmt"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -9,9 +11,31 @@ import (
 	"testing"
 	"time"
 
+	"github.com/klauspost/compress/zstd"
+
 	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/erf"
 )
 
+// TestSingleThreadedEncoderMatchesDefault pins the claim the blob encoder's
+// concurrency setting rests on: it saves memory only, and a published blob is
+// the same bytes either way.
+func TestSingleThreadedEncoderMatchesDefault(t *testing.T) {
+	standard, err := zstd.NewWriter(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer standard.Close()
+
+	body := make([]byte, 4<<20)
+	random := rand.New(rand.NewSource(1))
+	random.Read(body[:len(body)/2])
+	for _, size := range []int{0, 1, 4 << 10, len(body)} {
+		if !bytes.Equal(blobEncoder.EncodeAll(body[:size], nil), standard.EncodeAll(body[:size], nil)) {
+			t.Fatalf("%d bytes compress differently at concurrency 1", size)
+		}
+	}
+}
+
 // resourceSize is one payload in the memory fixtures. Real haks hold a few MB
 // per resource, and peak memory is meant to track that, not the archive.
 const resourceSize = 1 << 20