Validate manifest entry paths on deserialize (closes #61)

Untrusted .mf files were parsed with no path validation, so an entry like ../../etc/passwd reached filepath.Join against the checker's base path and mfer check could stat and read outside it. ValidatePath ran only when building a manifest. It now runs on every entry as the manifest loads, so every consumer is covered. The whole manifest is rejected on the first bad entry instead of dropping it, which could hide files from a check; the error wraps errInvalidManifestPath and names the path.

Disclosure: a path that is not valid UTF-8 is refused earlier, by the protobuf string decoder, so that error does not name the path.

Model: opus-4-8 (implementation); fable-5-1 (summary)

mfer/deserialize.go

@@ -25,6 +25,7 @@ var (
 	errDecompressedTooLarge = errors.New("decompressed data exceeds maximum allowed size")
 	errUUIDMismatch         = errors.New("outer and inner UUID mismatch")
 	errInvalidFileFormat    = errors.New("invalid file format")
+	errInvalidManifestPath  = errors.New("manifest contains invalid path")
 )
 
 // validateUUID checks that the byte slice is a valid UUID (16 bytes, parseable).
@@ -181,6 +182,19 @@ func (m *manifest) deserializeInner() error {
 		return errUUIDMismatch
 	}
 
+	// Enforce the manifest path invariants on every entry as it is loaded,
+	// so that no consumer of a manifest — Checker today, any restore or
+	// extract path tomorrow — acts on a traversal or absolute path from an
+	// untrusted .mf. Reject loudly on the first offender rather than
+	// dropping entries, which would let a hostile manifest hide files from a
+	// check.
+	for _, f := range m.pbInner.GetFiles() {
+		err = ValidatePath(f.GetPath())
+		if err != nil {
+			return fmt.Errorf("%w: %w", errInvalidManifestPath, err)
+		}
+	}
+
 	log.Infof("loaded manifest with %d files", len(m.pbInner.GetFiles()))
 
 	return nil