Refuse a path listed twice in a manifest (closes #170)

gen given arguments whose files share a path, such as gen a b with a.txt
in both, or gen . ., now fails while listing the files, before hashing
any, naming the path and both files. Builder.AddFile and
Builder.AddFileWithHash refuse a path already added. Loading refuses a
manifest that lists a path twice, compared byte for byte; fetch keeps
its own letter-case check. The Path Rules in docs/FORMAT.md say each
path appears at most once. The decode-size test listed one path 1000
times; each entry now has its own path of the same length.

Model: opus-5-5

docs/FORMAT.md

@@ -109,9 +109,12 @@ All `path` values must satisfy these invariants:
 - **No parent traversal**: 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
+  for byte, so `A.txt` and `a.txt` are two paths
 
 Implementations must validate these invariants when reading and writing
-manifests. Paths that violate these rules must be rejected.
+manifests. Paths that violate these rules must be rejected, and a reader must
+reject a manifest that lists a path more than once.
 
 ## Hash Format (`MFFileChecksum`)