BuildPositionToLineMap incremented position and updated maxPositions for
@@ hunk-header lines but did not store a map entry, causing Translate()
to return a hard error for any comment positioned at a hunk header.

Store sentinel value 0 for hunk-header positions (analogous to -1 for
deletions) and extend Translate() to fall through to the nearest
context/addition line below, matching the existing deletion-line
behavior.

Fixes #97

@@ -50,2 +52,4 @@
 			}
 		}
+		if lineNum == 0 {
+			return 0, fmt.Errorf("position %d targets a hunk-header line with no subsequent new-file line in %q", position, file)

@@ -275,0 +305,4 @@
+--- a/file.go
++++ b/file.go
+@@ -1,3 +1,3 @@ package main
+ line1

@@ -47,4 +49,4 @@
 		for p := position + 1; p <= maxPos; p++ {
 			if ln, exists := fileMap[p]; exists && ln > 0 {
 				return ln, nil
 			}

@@ -46,2 +46,4 @@
+	// Both map to the nearest context/addition line below.
+	if lineNum <= 0 {
 		maxPos := pm.maxPosition(file)
 		for p := position + 1; p <= maxPos; p++ {

@@ -48,4 +50,4 @@
 			if ln, exists := fileMap[p]; exists && ln > 0 {
 				return ln, nil
 			}
 		}

@@ -275,0 +302,4 @@
+	// Second hunk: @@ is pos 5, then " func foo() {" (pos 6), "+// added" (pos 7), etc.
+	// Translating position 5 (second @@) should resolve to new line 10.
+	diff := `diff --git a/file.go b/file.go
+--- a/file.go