rootio: fix GPT partition index and sector alignment in Partition()

[shcherbak]

Two related bugs in rootio/storage/partition.go's Partition() (GPT path) that together make rootio partition silently produce a broken or misaligned partition table:

1. gpt.Partition.Index is never set. go-diskfs's gpt.Partition requires a 1-based Index into its 128-entry
   GPT array (see table.toBytes() in go-diskfs). Fix: set Index: d.Partitions[x].Number when building each gpt.Partition.
2. Partition starts are not aligned to a sector/erase-block boundary. The loop computed the next partition's start as sectorEnd + 1, i.e. the very next 512-byte sector after the previous partition's end, with no regard for the underlying physical/logical sector size. On disks with 4Kn or 512e physical sectors (e.g. many enterprise/RAID controllers such as PERC/megaraid, which is what we hit this on) this produces partitions that don't start on a physical sector boundary, which fdisk/parted flag (Partition N does not start on physical sector boundary) and which can hurt performance or, on some controllers, cause issues with subsequent tooling that assumes conventional alignment. Fix: add an alignUp() helper that rounds a sector number up to the next 2048-sector (1MiB) boundary