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/builder.go

@@ -34,6 +34,7 @@ var (
 	errPathAbsolute     = errors.New("is absolute; must be relative")
 	errPathEmptySegment = errors.New("contains empty segment")
 	errPathDotDot       = errors.New("contains '..' segment")
+	errPathDot          = errors.New("contains '.' segment")
 	errSizeMismatch     = errors.New("size mismatch")
 	errNegativeSize     = errors.New("size cannot be negative")
 	errHashNotMultihash = errors.New("hash is not a valid multihash")
@@ -46,6 +47,7 @@ var (
 //   - Must use forward slashes only (no backslashes)
 //   - Must be relative (no leading /)
 //   - Must not contain ".." segments
+//   - Must not contain "." segments
 //   - Must not contain empty segments (no "//")
 //   - Must not be empty
 func ValidatePath(p string) error {
@@ -73,6 +75,10 @@ func ValidatePath(p string) error {
 		if seg == ".." {
 			return fmt.Errorf("path %q %w", p, errPathDotDot)
 		}
+
+		if seg == "." {
+			return fmt.Errorf("path %q %w", p, errPathDot)
+		}
 	}
 
 	return nil