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.

README.md

@@ -241,7 +241,16 @@ run would move rather than what it would delete.
 Rockbox's `MAX_PATH` is 260, from `firmware/include/fs_defines.h`, and it bounds
 the path *as the device sees it*. The directory the mirror is copied into comes
 out of the same budget, so `--device-prefix` (default `/Music`) is subtracted
-from `--max-path` to get what a mirror-relative path may spend.
+from `--max-path` to get what a mirror-relative path may spend:
+
+| Destination on the device | Mirror-relative budget |
+| ------------------------- | ---------------------- |
+| `/Music/` | 253 |
+| the card root | 259 |
+
+Worth being exact about, because a checker that measures mirror-relative paths
+against the flat 260 quietly passes everything from 253 to 260 — and those are
+the paths most likely to be near the edge in the first place.
 
 Over-budget paths are shortened from the **deepest component outward**: the
 track name carries the least navigational value and the artist directory the