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)",

test/system/services/block-range-test/block-range-test.zig

@@ -133,6 +133,12 @@ pub fn main(init: process.Init) void {
     // range, and must be refused.
     verdict(device.read(0, 1, bounce.physical), "in-range-read");
     verdict(!device.read(2, 1, bounce.physical), "out-of-range-refused");
+    // The wrap attack, calibrated to be exploitable against a naive bound: this
+    // process is confined with base 1, so a volume-relative LBA of maxInt(u64)
+    // makes base + lba wrap to absolute block 0 — a real, readable block OUTSIDE
+    // the range (the boot sector). A naive `lba + count > count` check also
+    // wraps to 0 and waves it through; the overflow-safe bound refuses it.
+    verdict(!device.read(std.math.maxInt(u64), 1, bounce.physical), "wrap-refused");
 
     // Geometry now reports the CONFINED size, not the device's.
     const confined = device.geometry() orelse {