Rethink the dated sena-3 snapshot test pattern

The dated sena-3 snapshot files (FwLiteProjectSync.Tests/Snapshots/sena-3_snapshot.<date>.verified.txt) make the newest file do two jobs that fight each other:

• AssertSena3Snapshots treats every dated file as a frozen historical fixture (prove old formats still deserialize) — so it should never change.
• LatestSena3SnapshotRoundTrips round-trips the newest one and verifies against that same filename — so it wants to track the current format.

When the serialization format changes, you either edit the newest dated file in place (breaks job 1, and the date becomes a lie) or remember to hand-add a brand-new dated file (job 2). Nothing fails if you pick wrong, so the historical guard can silently rot. (This already happened once with the IsMain change.)

Likely fix: split the two jobs. Have an un-dated sena-3_snapshot.latest.verified.txt as the round-trip target (legitimately mutable, always "current"), and keep the dated files as deserialize-only inputs that are never edited. The sibling SnapshotDeserializationRegressionData.latest/.legacy tests in the same project already do this — worth borrowing the shape.

Start in FwLiteProjectSync.Tests/Snapshots and locate AssertSena3Snapshots, LatestSena3SnapshotRoundTrips, and the dated sena-3 snapshot files. Read the sibling SnapshotDeserializationRegressionData.latest/.legacy tests for the existing pattern; done means the latest round-trip uses an undated mutable fixture while dated files remain deserialize-only historical inputs.