[sandbox audit] Recycle sandbox uids, but only when teardown is verified

[Wauplin]

[sandbox audit] — PR 9 of 13 in this repo's stack; merge in order.
Previous: #22 · Next: #25
Review only the commits this PR adds on top of its base; bases collapse to main as the stack lands.

Why

Uids were allocated monotonically from 20000 and never reused. A host that created ~45,000
sandboxes over its 24h lifetime could no longer create one even while empty.

Worse than the exhaustion itself: it surfaced as a 500, which the client reads as a hard
error rather than "this host is full" — so the sandbox wasn't re-placed onto another host, the
create() just failed.

Approach

Reuse is the fix, and a careless free list would be worse than the problem: hand back a uid
whose processes are still alive and the next sandbox inherits them, with the ability to signal
and read them. So:

• a uid is freed only when teardown is known to have converged — which the previous PR's
  kill_uid return value makes knowable;
• anything doubtful is quarantined for the process's lifetime: surviving processes, or a
  home directory that couldn't be removed (files owned by that uid may remain);
• and it is re-checked at hand-out, because a decision made a second ago is not a
  guarantee about now.

Losing one uid out of 45,000 is the cheap side of that trade.

Exhaustion now reports CreateError::Full, so the client packs elsewhere.

Also: at startup, sample the uids the running image already uses in our range — from
/etc/passwd and from any process already running under one — and reserve them. UID_BASE = 20000 was chosen on the assumption that images don't use it, but nothing checked, and a
collision would put two "isolated" sandboxes under a single uid, defeating the entire DAC half
of the isolation model.

Validation

• 48 unit tests, including: reuse ordering; quarantine honoured from either direction;
  image-reserved uids skipped; exhaustion stable rather than intermittent; UID_BASE/UID_MAX
  staying inside the container's 0..65535 uid map; and that a "free" uid which still owns
  processes is quarantined at hand-out rather than reused (using our own uid, which certainly
  has one).

• Live, root container:

  12 sequential create/delete cycles -> uids handed out: 20000 (×12), distinct: 1
  4 concurrently live sandboxes      -> distinct uids: 4
  

  Both directions matter: recycling that also collided between live sandboxes would be a far
  worse bug than the one being fixed.

Depends on

The honest kill_uid return value from #21 — without it there is no way to know a uid is safe
to reuse, and this PR would be the dangerous kind of fix.