ci: tag DAT-dependent tests into the InstalledDat lane; drop invariant workaround

Three tests reached the CI gate needing the installed retail DATs, which no
build machine has, and failed with FileNotFoundException on client_cell_1.dat:
  - Issue127FloodFlipReplayTests (both facts replay via ResolveDatDir)
  - FindCellListConformanceTests.FindCellList_DoorwayThreshold_IndoorPicks_
    MatchRetail, the one untagged method among already-tagged siblings
They now carry [Trait("Lane", "InstalledDat")] like every other DAT test, so
the gate filter excludes them and the local DAT lane still runs them.

Also reverts the DOTNET_SYSTEM_GLOBALIZATION_INVARIANT pin from the previous
commit. It was too blunt: it fixed the 40 decimal-comma failures but broke
ChatLogTests.FormatTimestampPrefix_UsesLiteralColons_RegardlessOfCurrentCulture,
which legitimately constructs a culture and cannot under invariant mode. The
runner's HKCU locale (LocaleName=en-SE, sDecimal=',') was corrected to en-US
instead, which is the actual defect.

tests/AcDream.Core.Tests/Conformance/FindCellListConformanceTests.cs

@@ -130,6 +130,7 @@ public class FindCellListConformanceTests
     /// separate outdoor-entry validation.
     /// </summary>
     [Fact]
+    [Trait("Lane", "InstalledDat")]
     public void FindCellList_DoorwayThreshold_IndoorPicks_MatchRetail()
     {
         var (cache, picks) = LoadThresholdGolden();