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 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

docs/FORMAT.md

@@ -107,6 +107,7 @@ All `path` values must satisfy these invariants:
 - **Forward slashes**: use `/` as the path separator (never `\`)
 - **Relative only**: no leading `/`
 - **No parent traversal**: no `..` path segments
+- **No current-directory segments**: no `.` path segments
 - **No empty segments**: no `//` sequences
 - **No trailing slash**: paths refer to files, not directories
 - **Listed once**: each path appears at most once in a manifest, compared byte