compress: expose Padmé size obfuscation via UI

Fixes #9286.
Tested manually via:

 = export BORG_REPO=repo
 = alias b='python -m borg'
 = echo meow > file
 = b create ::test ./file -C obfuscate,250,none
 = b info
Archive name: ::test
Archive fingerprint: 2c67ba6c04faa6d911b5dd02b99593ea2cf6ebe7c38e2e6fca90afad47a7095e
[...]
Command line: /home/user/c/d0/software/backup/borg/src/borg/__main__.py create ::test ./file -C obfuscate,250,none
[...]
Number of files: 1
Original size: 40 B
 = rm file
 = b list ::test
-rw-r--r-- user   users         5 Mon, 2026-02-09 22:41:54 +0100 file
 = b extract ::test file
 = cat file
meow
 = # Yup, all good. Let's go for something nontrivial to make sure it's stripped accordingly.
 = dd if=/dev/urandom of=file bs=1M count=4
 = b create ::test ./file -C obfuscate,250,zstd
 = mv file original
 = b info
Archive name: ::test
Archive fingerprint: 2c67ba6c04faa6d911b5dd02b99593ea2cf6ebe7c38e2e6fca90afad47a7095e
[...]
Command line: /home/user/c/d0/software/backup/borg/src/borg/__main__.py create ::test ./file -C obfuscate,250,none
Working Directory: /home/user/l/bug/borg
Number of files: 1
Original size: 40 B

Archive name: ::test
Archive fingerprint: 01014345dd1187f014f19ac59a2313a4e6d644532b287d806fe87882ca47ea0d
[...]
Command line: /home/user/c/d0/software/backup/borg/src/borg/__main__.py create ::test ./file -C obfuscate,250,zstd
Working Directory: /home/user/l/bug/borg
Number of files: 1
Original size: 4.19 MB
 = b extract aid:01014345dd1187f014f19ac59a2313a4e6d644532b287d806fe87882ca47ea0d 
 = sha256sum file original
d74be01406764ef11f5aa3ee27033bdced2c9573c0b3ed980fcfb2116c7d84de  file
d74be01406764ef11f5aa3ee27033bdced2c9573c0b3ed980fcfb2116c7d84de  original
 =

src/borg/compress.pyx

@@ -713,8 +713,8 @@ class CompressionSpec:
         elif self.name == 'obfuscate':
             if 3 <= count <= 5:
                 level = int(values[1])
-                if not ((1 <= level <= 6) or (110 <= level <= 123)):
-                    raise ArgumentTypeError("level must be >= 1 and <= 6 or >= 110 and <= 123")
+                if not ((1 <= level <= 6) or (110 <= level <= 123) or (level == 250)):
+                    raise ArgumentTypeError("level must be (inclusively) within 1...6, 110...123 or equal to 250")
                 self.level = level
                 compression = ','.join(values[2:])
             else: