fix: cost the device prefix exactly rather than approximately

The budget subtracted the prefix length plus two, on the assumption of a
leading and a trailing slash. That is right for /Music and wrong for an empty
prefix, where there is only one slash -- losing a character at the card root,
which is exactly where the longest paths sit.

Computed from the prefix as it will actually appear instead: /Music/ costs
seven characters and gives a mirror-relative budget of 253, the root costs one
and gives 259.

Worth being exact about because the reverse error is worse. A checker
comparing mirror-relative paths against the flat 260 passes everything between
253 and 260, and those are precisely the paths closest to the edge.

music_mirror.py

@@ -167,6 +167,18 @@ def fat32_safe(component):
     return cleaned or "_"
 
 
+def device_prefix_length(prefix):
+    """Return the on-device prefix as it will actually appear, with slashes.
+
+    "/Music" costs seven characters -- the leading slash, the name, and the
+    separator before the mirror's own path -- while an empty prefix costs one.
+    Approximating that loses a character at the root, which is precisely where
+    the longest paths are.
+    """
+    cleaned = prefix.strip("/")
+    return f"/{cleaned}/" if cleaned else "/"
+
+
 def shorten_component(component, budget):
     """Return a component of at most `budget` characters, cut from the middle.
 
@@ -783,7 +795,7 @@ def main(argv=None):
 
     # The device's limit covers the whole path it will see, so what the mirror
     # may spend is that less the directory it gets copied into.
-    budget = max(0, args.max_path - len(args.device_prefix.strip("/")) - 2)
+    budget = max(0, args.max_path - len(device_prefix_length(args.device_prefix)))
     if args.fat32_safe:
         logger.info(
             "paths are limited to %d characters, from --max-path %d less the %r prefix",