block: close the range-clamp overflow — a confined caller could wrap into the neighbour

The naive bound `lba + count > r.count` wraps for an lba near u64 max: the
sum overflows to a small value, sails under the check, and `base + lba`
wraps to an absolute block OUTSIDE the range. Calibrated, it is a real
confinement escape — a process confined to [1,3) reads absolute block 0
(the boot sector) with lba = maxInt(u64), since base + lba wraps to 0.

The bound is rewritten as two subtractions that cannot overflow: lba within
the range, and count within what remains. block-range gains a wrap-refused
assertion calibrated to be exploitable against the naive form — it FAILS
against the old bound (reads block 0) and passes against the fix (verified
by reverting the clamp). Caught pre-emptively before the V2 boundary review.

test/qemu_test.py

@@ -1246,6 +1246,7 @@ CASES = [
      "timeout": 150,
      "expect": r"(?s)(?=.*block-range: ok in-range-read)"
                r"(?=.*block-range: ok out-of-range-refused)"
+               r"(?=.*block-range: ok wrap-refused)"
                r"(?=.*block-range: ok geometry-is-confined)"
                r"(?=.*block-range: ok confined-cannot-redefine)"
                r"(?=.*block-range: VERDICT done)",