feat(phase-b): GameSession snapshot save/load (JSON)

ToJson/FromJson round-trip the persistent global bank; play --save-state <file>
persists it, --state <file> restores it — so an expensive booted state (23646
globals) is captured once and reused without re-running *INIT, and it's the
foundation for real save-file work. Tests: unit round-trip + booted-state
survives snapshot (skill data intact). Engine 18/18.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

engine/Age.Engine.Tests/GameSessionTests.cs

@@ -90,6 +90,30 @@ public class GameSessionTests
         Assert.True(session.Globals.Count > 1000, $"SKINIT should populate the skill table (got {session.Globals.Count})");
     }
 
+    [Fact]
+    public void SnapshotRoundTripsState()
+    {
+        var s = new GameSession();
+        s.Seed(0x10, 42); s.Seed(0x20, -7); s.SeedString(0x30, "リリィ");
+        var t = GameSession.FromJson(s.ToJson());
+        Assert.Equal(42, t.Globals[0x10]);
+        Assert.Equal(-7, t.Globals[0x20]);
+        Assert.Equal("リリィ", t.GlobalStrings[0x30]);
+        Assert.Equal(s.Globals.Count, t.Globals.Count);
+    }
+
+    [Fact]
+    public void BootedStateSurvivesSnapshot()
+    {
+        var s = new GameSession();
+        s.RunScene(Sys4Loader.Load(Paths.Scripts()["SKINIT.BIN"], Table), Table, new CaptureHost());
+        var t = GameSession.FromJson(s.ToJson());   // snapshot the expensive booted state, rebuild
+        Assert.Equal("飛行", t.GlobalStrings[0x23a3]);
+        Assert.Equal(30, t.Globals[0xa6e5b]);
+        Assert.Equal(s.Globals.Count, t.Globals.Count);
+        Assert.Equal(s.GlobalStrings.Count, t.GlobalStrings.Count);
+    }
+
     [Fact]
     public void SeedingFormFlagChangesBehavior()
     {