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.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;