v1.46.4: detached plans were being collected as garbage — data loss

Deployed v1.46.3 to my own instance, restarted, and the startup sweep deleted
both floor plans: config/houseplan/plans/ went from f1.svg + f2.png to empty.
The backup is a SecureTar, so they are gone.

The rule was wrong, not the code. v1.46.0 introduced collection that treats
'nothing references this right now' as abandoned and gives it an hour. But
detaching a plan — switching a space to 'draw' — is a normal, reversible action,
and the editor's own comment says the file stays on disk. Those two plans had
been detached for weeks; every pass since v1.46.0 was entitled to remove them,
and the one that finally ran did.

New rule, one for every path:
  * superseded by a commit (was in the old revision, is not in the new) — goes
    immediately; that is the one thing a commit knows for certain;
  * belongs to a space or marker that still exists — never collected, at any
    age, because unreferenced is not abandoned;
  * a per-dialog staging folder (up_*) — one hour, unchanged: by construction it
    only ever holds an upload from a dialog that was never saved;
  * anything else — thirty days.

The  flag I added an hour ago is gone with it: two rules for the same
question is how this happened. Tests updated to the new grace, plus two that pin
the distinction directly.

I am sorry about the files.

docs/TESTING.md

@@ -239,6 +239,10 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       on the plan after a reload. Same for each tap action and each fill mode
       [auto: backend test_every_display_mode_the_editor_offers_is_accepted and
       neighbours, test_a_marker_showing_its_value_can_be_saved]
+- [ ] Detaching a plan keeps the file (v1.46.4): switch a space to "draw",
+      restart, wait a day — the image is still in `config/houseplan/plans/` and
+      can be re-attached. Replacing a plan still removes the one it replaced,
+      immediately [auto: unit: test_scheduled_collection_never_takes_a_detached_plan]
 - [ ] Nothing accumulates on an idle instance (v1.46.2/v1.46.3, HP-1461-01,
       HP-1462-01): attach a file, cancel the dialog, and do not save anything
       else — the file is gone after a restart AND after the daily pass, while