fix(volume): 🐛 use overflow-safe arithmetic in bounds checks

[justapithecus]

Summary

• Rewrite all offset + length > limit patterns as length > limit - offset to prevent silent int64 overflow when offset is near math.MaxInt64
• Affects 5 locations in volume.go: StageWriteAt, Commit, ReadAt, validateNoOverlaps, validateVolumeManifest
• Add 7 overflow-specific tests exercising near-MaxInt64 values across all affected code paths

Detail

Go's int64 arithmetic wraps silently on overflow. A malformed BlockRef with Offset = math.MaxInt64 - 5, Length = 10 would cause Offset + Length to wrap to a small positive number, passing the bounds check offset + length > totalLength incorrectly.

The fix rewrites each check as length > totalLength - offset, which is overflow-safe because both offset and totalLength are validated non-negative upstream.

For validateNoOverlaps, the overlap check prevEnd > sorted[i].Offset (where prevEnd = sorted[i-1].Offset + sorted[i-1].Length) is rewritten as sorted[i-1].Length > sorted[i].Offset - sorted[i-1].Offset, safe because the slice is sorted in ascending offset order.

Test plan

• All 7 new overflow tests pass
• All existing Volume tests still pass
• golangci-lint clean

🤖 Generated with Claude Code