No fixture exercises GNU root registration from the header pair (a fact line always supplies it first) #3925

A GNU diff -r stream teaches the parser which roots diff was given, so that a bare
(diffutils 3.10) Only in <dir>: <file> whose text holds more than one : splits at a
root the stream already named rather than at the first : .

Two places register those roots: the ---/+++ header pair (gnu_root_set.push under
if gnu_roots && !hg_stream) and the GNU fact arms (Binary files / Only in). Every
fixture that exercises this carries a fact line, so the header-pair registration is never
the one doing the work — and disabling it entirely fails no test.

Verified on develop (665b272) by removing the ! from the header-pair guard so the set
is never populated there, then running two inputs:

# shipped fixture — has "Binary files d: x/b.bin and e: y/b.bin differ" before the Only in
tests/fixtures/diff/diff_ru_310_colon_dirs_raw.txt
  correct : [file] e: y/only.txt (only in one side)
  patched : [file] e: y/only.txt (only in one side)   <- fact arm supplied the root

# same shape with no fact line, so only the header pair can name the root
diff -ru d: x/f.txt e: y/f.txt
--- d: x/f.txt	2026-09-07 13:03:55.284360030 +0000
+++ e: y/f.txt	2026-09-07 13:03:55.284360030 +0000
@@ -1 +1 @@
-a
+b
Only in e: y: only.txt

  correct : [file] e: y/only.txt (only in one side)
  patched : [file] e/y: only.txt (only in one side)   <- split at the first ": "

The second input is the missing fixture: a GNU echo, a header pair and a multi-:
Only in, with no Binary files / Files line ahead of it to supply the root. It is a
shape real diff -ru produces whenever the differing files under such a directory are all
text.

The behaviour is correct today; this is purely a hole in the corpus. Found while triaging
mutation-test survivors for #3923, which covers the other gaps that turned up.