feat(samples): three devices sharing one database, with no server between them

[pal-tamas]

Three devices — Phone, Laptop, Tablet — each with its own SQLite file and its own replica identity,
sharing a bucket and nothing else. Normally these would be three phones; here they are three files in
one process, which is the only difference that matters. Item 7 of #642.

Stacked on #666 (which is stacked on #665). Retarget as those merge.

RASK_CRSQLITE_PATH=/path/to/crsqlite.dylib dotnet run --project samples/Rask.Example.Crdt

The thing to try

Take two devices offline, edit different fields of the same todo on each — the priority on one, the
done flag on the other — bring both back and press Sync everyone. Both edits survive. Do the same with
a LastModified column and one of them is gone.

An E2E drives exactly that through a browser, so the claim is tested rather than asserted in prose.

Each device's link to the bucket has a switch, and flipping it makes every call fail as a real client
would see it — so the demo exercises the real offline path rather than a case the engine knows about.
The status line says "your edits are saved and will sync later" rather than reporting a failure,
because the edit is already committed to that device's own database.

FolderObjectStore

The same IObjectStore over a directory. It is what lets the sample run with no cloud credentials, but
it is not only a test double:

• a single-machine deployment with no reason to pay for object storage;
• a folder something else already replicates — pointed at a Syncthing share, devices converge with
  no central server at all; pointed at iCloud Drive or Dropbox, the replication is somebody else's
  problem.

Objects are written beside their key and moved into place, so a reader listing concurrently sees either
nothing or the whole object — which matters most when another process is replicating the folder while it
is being written. Keys that would escape the root are refused rather than normalised, since a key can
come back from a listing of a folder other people also write to. TryCreateAsync maps to the
filesystem's own atomic create, so the conditional-create pattern works there too.

The missing-binary problem, handled honestly

cr-sqlite ships per-platform and is not redistributed here. Without RASK_CRSQLITE_PATH the page
explains what to download instead of failing at the first query — a missing extension otherwise surfaces
as no such function: crsql_as_crr, which says nothing about what to do.

That state is asserted too, so the E2E file never quietly skips in full on a machine without the
binary; one of the two paths always runs. Verified by mutation: breaking the setup-card assertion turns
it red on a no-extension run, so it is not passing vacuously.

Testing

dotnet test tests/Rask.ObjectStore.Tests                  # 78 passed (16 new)
RASK_CRSQLITE_PATH=… <pre-push E2E gate>                  # 64 browser journeys (was 60)

The four new journeys ran inside the real gate with the extension present, and pass without it via the
setup-card path.

Local gates: dotnet format clean, dotnet build -warnaserror clean, 336 + 320 unit, 64 browser
journeys, 26 CLI build-gate.

A note on the build

Building the E2E project tripped the MSBuild node-reuse bake failure (#650) — the guard added in
#652 caught it and named the fix (-nodeReuse:false). Pre-existing, not introduced here, but worth
knowing it is still reachable from an ordinary project build.