Refuse a path with a . segment (closes #208)

ValidatePath refused a .. segment but accepted a . segment, so a
manifest could list ./a.txt. check then verified a.txt and also
reported it as an extra file, and one file could be listed twice under
two spellings despite the listed-once rule. A . segment is now refused
when a manifest is written and when it is loaded, and the Path Rules
in docs/FORMAT.md and the path comment in mfer/mf.proto list it.

Two fetch tests listed ./ paths to check name clashes; such manifests
can no longer be built, so those cases are removed.

Model: opus-5-5

mfer/mf.pb.go

@@ -333,8 +333,8 @@ type MFFilePath struct {
 	state protoimpl.MessageState `protogen:"open.v1"`
 	// required attributes:
 	// Path invariants: must be valid UTF-8, use forward slashes only,
-	// be relative (no leading /), contain no ".." segments, and no
-	// empty segments (no "//").
+	// be relative (no leading /), contain no ".." or "." segments, and
+	// no empty segments (no "//").
 	Path string `protobuf:"bytes,1,opt,name=path,proto3" json:"path,omitempty"`
 	Size int64  `protobuf:"varint,2,opt,name=size,proto3" json:"size,omitempty"`
 	// gotta have at least one:

mfer/mf.proto

@@ -49,8 +49,8 @@ message MFFileOuter {
 message MFFilePath {
     // required attributes:
     // Path invariants: must be valid UTF-8, use forward slashes only,
-    // be relative (no leading /), contain no ".." segments, and no
-    // empty segments (no "//").
+    // be relative (no leading /), contain no ".." or "." segments, and
+    // no empty segments (no "//").
     string path = 1;
     int64 size = 2;
 

mfer/mf.proto.sha256

@@ -1 +1 @@
-4f8ea5276a8e66590e71ed154a9176b0f68ffa9ab69d8de5aff0e0d9b731164d  mf.proto
+1668b2f6b8639a9213873474e58dbacb3a4ef77cc27c42253cad8dd076e5f206  mf.proto