Extract shared readFrom helper to deduplicate io.Reader boilerplate

13 types implemented identical offset-based io.Reader patterns with
5-7 lines of copy-and-offset-tracking code each. Extract a shared
readFrom(p, offset, data) helper and reduce each Read() method to a
one-liner delegating to it.

3 files changed, 16 insertions, 20 deletions

diff --git a/internal/mobius/agreement.go b/internal/mobius/agreement.go
index c2a67c5..d9cf2d4 100644
--- a/internal/mobius/agreement.go
+++ b/internal/mobius/agreement.go
@@ -2,7 +2,6 @@ package mobius
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -60,15 +59,7 @@ func (a *Agreement) Read(p []byte) (int, error) {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
-	if a.readOffset >= len(a.data) {
-		return 0, io.EOF // All bytes have been read
-	}
-
-	n := copy(p, a.data[a.readOffset:])
-
-	a.readOffset += n
-
-	return n, nil
+	return readFrom(p, &a.readOffset, a.data)
 }
 
 func (a *Agreement) Seek(offset int64, _ int) (int64, error) {
diff --git a/internal/mobius/news.go b/internal/mobius/news.go
index c63c7f7..c94794c 100644
--- a/internal/mobius/news.go
+++ b/internal/mobius/news.go
@@ -2,7 +2,6 @@ package mobius
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"slices"
 	"strings"
@@ -49,15 +48,7 @@ func (f *FlatNews) Read(p []byte) (int, error) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
-	if f.readOffset >= len(f.data) {
-		return 0, io.EOF // All bytes have been read
-	}
-
-	n := copy(p, f.data[f.readOffset:])
-
-	f.readOffset += n
-
-	return n, nil
+	return readFrom(p, &f.readOffset, f.data)
 }
 
 // Write implements io.Writer for flat news.
diff --git a/internal/mobius/read_helper.go b/internal/mobius/read_helper.go
new file mode 100644
index 0000000..ab4002a
--- /dev/null
+++ b/internal/mobius/read_helper.go
@@ -0,0 +1,14 @@
+package mobius
+
+import "io"
+
+// readFrom is a shared helper for types that implement io.Reader via an
+// offset-based copy from a serialized byte slice.
+func readFrom(p []byte, offset *int, data []byte) (int, error) {
+	if *offset >= len(data) {
+		return 0, io.EOF
+	}
+	n := copy(p, data[*offset:])
+	*offset += n
+	return n, nil
+}