v1.46.1: re-check of v1.46.0 — HP-1460-01, -02, -03

HP-1460-01: v1.46.0 stopped overwriting attachments, but picking a free name
and taking it were two steps. Two uploads racing between them agreed on the
same name, both answered 200, and one set of bytes replaced the other;
files/migrate had the same check-then-copy gap. reserve_filename now claims the
name with O_CREAT|O_EXCL as it picks it, and both paths use it. It also splits
the extension off the RAW name and budgets the stem against MAX_FILENAME
including the collision tag — a maximal name lost its '.pdf' and then grew past
the limit, so the view sanitised the request back to a different name and the
attachment 404'd for good.

HP-1460-02: cleanup lived in an 'except Exception', which CancelledError walks
past, only one tmp_path was tracked, promotion had no finally, and the
collector only walks marker folders — an aborted transfer stranded a .upload-*
that nothing would ever remove. An outer finally owns every temporary, a second
'file' part is refused, promotion failure cleans up, and sweep_upload_temps
runs at setup, daily, and inside the commit-scoped collector. Chunks are
batched to 1 MB per disk task instead of one per 64 KB.

HP-1460-03: the layout event reached the static card and not the full one, so
two full cards diverged until a reload. The full card subscribes now and
re-reads ONLY the layout, keyed on its revision. Two hazards handled: it
records revisions it produced itself, and the reaction is deferred ~200 ms
because the event can beat the reply to our own write over the same socket;
positions dragged but not yet sent are flushed and merged on top, so a fix for
a stale UI cannot become a lost drag.

Tests: smoke_layout_sync (fails on a v1.46.0 build), four pure tests for atomic
reservation incl. 20-thread concurrency and the length boundary, a backend test
walking every failing exit path of an upload, and — as the report asked — an
HA-harness test that a repair issue disappears with its space.
Docs: CHANGELOG.md + CHANGELOG.ru.md + ARCHITECTURE.md + TESTING.md + STATUS.md.

docs/ARCHITECTURE.md

@@ -190,9 +190,16 @@ and only with SVG: a CSP on a PDF response can break the browser's built-in
 viewer, and a raster image has no execution model to disable.
 
 **Attachments follow the same commit-scoped lifecycle as plans** (HP-1454-02).
-An upload takes a free name (`unique_filename`) and never overwrites, because
-the bytes under an existing name may be referenced by the stored configuration
-and an upload is not part of that transaction. A new icon has no id yet, so its
+An upload takes a free name and never overwrites, because the bytes under an
+existing name may be referenced by the stored configuration and an upload is
+not part of that transaction. `reserve_filename` *claims* the name as it picks
+it (`O_CREAT | O_EXCL`) — asking `exists()` and returning a string let two
+uploads agree on one name and quietly overwrite each other. It also budgets the
+length so the result survives the sanitiser the content view applies to the
+request, since a name the view rewrites is a file written and never served.
+Streaming temporaries live in the files root under `.upload-`, are removed on
+every exit path of the request, and are swept at startup and daily for the
+cases a process death leaves behind. A new icon has no id yet, so its
 files go to a per-dialog staging folder and move to the real id once the config
 write is accepted — the same copy → save → cleanup order as a rebind.
 `config/set` collects what its commit superseded, and anything unreferenced and