vmm: one-shot mode allocates CIDs outside the IdPool and is invisible to the VMM

## Summary

One-shot mode (`vmm/src/one_shot.rs`) and the main VMM service allocate vsock CIDs from the same configured range through two mechanisms that do not know about each other. Nothing prevents them from picking the same CID.

## The two allocators

**Main service** — `IdPool` over `[cid_start, cid_start + cid_pool_size)`, rebuilt on reload from the supervisor's process list (`app.rs`, `reload_vms` / `reload_vms_sync`).

**One-shot** — `one_shot.rs:24-56`:

```rust
// scan `ps aux` for qemu-system-x86_64 ... guest-cid=
let mut one_shot_cid = config.cvm.cid_start;
while existing_cids.contains(&one_shot_cid) {
one_shot_cid += 1;
...
}
```

It starts at `cid_start`, avoids collisions by scraping `ps aux`, and never touches the pool.

## Why they can collide

One-shot launches QEMU directly (`cmd.status()` at the end of `run_one_shot`) rather than registering the process with the supervisor. `occupied_cids` in both reload paths is built from `supervisor.list()`, so **a one-shot VM's CID is invisible to the main service** and never gets occupied in the pool.

The blindness is one-directional:

| | sees the other's CIDs? | via |
|---|---|---|
| one-shot → main service | yes | `ps aux` finds the qemu processes |
| main service → one-shot | **no** | one-shot never reaches the supervisor |

So the main service can allocate a CID that a running one-shot VM already holds.

Two secondary issues in the same code path:

- **TOCTOU** — the `ps aux` scan and the QEMU launch are not atomic; a concurrent allocation in the window collides regardless.
- **Parsing** — CIDs are recovered by string-splitting `ps aux` output on `guest-cid=`, which is sensitive to how QEMU arguments are formatted.

## Note on #907

Before #907, `IdPool::allocate()` had an off-by-one that made it skip `cid_start` entirely, while one-shot starts *at* `cid_start`. That incidentally kept the two apart. #907 fixed the off-by-one (correctly — `one_shot.rs:48` and `crates/dstackup/src/cid.rs` both already treat the window as `[start, start+size)`), which removes the accidental separation.

**This is not a regression introduced by #907.** The protection only ever held for exactly one one-shot VM: a second one takes `cid_start + 1`, which was already inside the main pool's allocation range. The underlying problem is that the two allocators were never coordinated.

## Possible directions

1. Register one-shot processes with the supervisor so the existing pool machinery covers them.
2. Reserve a dedicated range for one-shot outside `[cid_start, cid_start + cid_pool_size)`.
3. Have one-shot allocate through `IdPool` rather than `ps aux`.

(1) seems most consistent with how the rest of the system tracks VMs, but one-shot is deliberately a lighter path, so (2) may be the cheaper fix.

## Confidence

The code paths are confirmed by reading: one-shot starts at `cid_start`, does not register with the supervisor, and both reload paths source `occupied_cids` from `supervisor.list()` only. **Not verified on hardware** — I have not observed an actual vsock CID collision, and I do not know how much one-shot mode is used in practice, which bounds how much this matters.

Found while reviewing #907.

Start with vmm/src/one_shot.rs, especially run_one_shot and the CID scan, then trace app.rs reload_vms/reload_vms_sync and crates/dstackup/src/cid.rs. Compare the proposed coordination approaches and determine how one-shot CIDs should be represented to the existing allocation machinery. Done means concurrent one-shot and main-service allocations cannot select the same CID.

Written by the indexing model from the issue text.