fix(desk): "everything else" is last UPDATED first, not last activity

The operator, on the live Desk: "how is this last activity first?" It was not,
usefully. The section sorted by `_newest_mtime`, which counts a look (`.viewed`),
so opening a booth moved it up. Tonight two post-deploy checks fetched every
booth page within half a second, which recorded 22 looks at once and collapsed
the section into reverse name order through the (mtime, name) tie-break.
Meanwhile each row shows "updated X ago", which is `landed_at`, a different
clock from the one the list was sorted by.

Operator ruling: "last activity can just be last time the booth was updated,
not necessarily operator's last activity." The section now sorts by
`(-landed_at, name)`, the date the row shows, labelled "last updated first".
Looking, flagging and blurring no longer move a booth. `list_booths` keeps its
own order for its other readers, and `_newest_mtime` still feeds lifetime.

The r2_flow contract (§3, the ordering table, INV-5) and ROADMAP's ordering row
are amended to match. Two tests and two r2_flow.toml rows cover it (25/25).

ROADMAP.md

@@ -105,7 +105,7 @@ Where it already binds, and what the rule is in each case:
 | the Desk's sections | fixed: needs you → new since you looked → everything else (R2) |
 | within *needs you* | `(open_since, name)` |
 | within *new since you looked* | `(-landed_at, name)` |
-| within *everything else* | `list_booths` order: `(mtime, name)` descending |
+| within *everything else* | `(-landed_at, name)`: last updated first, the date each row shows. Was `list_booths`' activity order, which counted a look (operator, 2026-09-23) |
 | the Desk's bookmarks column | `order_for_display` — pinned first, then newest |
 | caption sidecar resolution | sorted scan, so two media files sharing a stem resolve the same way every time (a real non-determinism U1 removed) |
 | marks in a booth | `(created, id)` — time, with the id as tie-break so two marks written in the same second cannot swap |