Add idempotency

[deitch]

When running through steps, first see if step previously took.

1. When shrinking filesystem, see if filesystem already is target size; if so, skip
2. When shrinking partition, see if partition already is target size; if so, skip
3. When creating new larger partitions, see if they match target ones; if so, skip
4. When copying contents to new partitions, first see if it already is copied; if so, skip

[eriknordmark]

Reading through resize.go and discover.go: a few observations on what the four-step list above does and doesn't cover, with code references.

Already in place (since discover.go populates partitionData.size fresh from p.GetSize() at every Run() invocation):

• shrinkFilesystems / shrinkPartitions correctly skip when current ≤ target (r.original.size <= r.target.size). Items 1 and 2 are already idempotent at the Run() level.
• createPartitions skips re-creating an alt-labeled partition via labelMap[altName] at resize.go:100 — covers item 3, with the caveat that the check is label-only (a previous half-create with the right label but wrong start/size would be silently kept).

Genuinely missing:

• copyFilesystems has no internal idempotency check (item 4). CompareFS is called after the copy, so on a retry that should have been a no-op, the full copy still happens.

Two more cases not on the list:

• swapPartitions is its own inverse — running it twice on the same in-memory resize plan undoes the swap. Not a problem when each Run() re-plans from scratch (because the second plan won't include a swap), but anyone reusing a plan across calls would be exposed.
• All per-phase functions (shrinkFilesystems, createPartitions, copyFilesystems, swapPartitions, removePartitions) are unexported, so a caller that wants to gate individual phases on its own preconditions has to either call the all-in-one Run() and trust planResizes to handle intermediate states (untested for partial completion), or fork. Exporting them — or providing a per-phase entry point alongside Run() — would close that gap.

[deitch]

Those are solid. I have no immediate plans to add those, but happy to take PRs on it.