fix(vm): faithful headless — halt at wait-for-input instead of plowing

The headless divergence that sent us chasing a phantom 'sleep' spin: op 0x72
wait-for-input was a no-op headless, so a run plowed past all 166 of a scene's
prompts into code no real playthrough reaches (SC0000 -> the name-entry poll
loop, spinning sleep 1 493k x to STEP-LIMIT). That path is a fiction.

Fix: VmOptions.HaltAtWaitForInput -> the VM halts (reason 'wait-for-input') at
0x72. run/play default to faithful (SC0000 now halts at ~402 steps, 0 sleeps,
matching the real run's path to the first prompt); --plow opts into the old
walk-every-page coverage. sweep stays plow by default (dialogue oracle, 284/13
unchanged); --halt-at-wait makes all 297 scenes halt cleanly at their first
prompt (0 STEP-LIMIT). Godot unaffected (really blocks on input; flag false).

Engine 58/58 (2 new); sweep default 284/13 unchanged; Godot selftest OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/HaltAtWaitTests.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using Age.Engine.Model;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
+using Xunit;
+
+public class HaltAtWaitTests
+{
+    private static OpcodeTable Table() => OpcodeTableJson.Load(Paths.OpcodesJson);
+
+    // Two-page scene: show A ; wait-for-input ; show B ; exit.
+    private static Age.Engine.Model.Script TwoPage(OpcodeTable t) => ScriptAssembler.Assemble(t, "TWOPAGE",
+        new List<(int, Operand[])>
+        {
+            (0x6e, new[] { new Operand(2, 0), new Operand(0, 0) }),   // show-text A
+            (0x72, new[] { new Operand(0, 0) }),                       // wait-for-input
+            (0x6e, new[] { new Operand(2, 1), new Operand(0, 0) }),   // show-text B
+            (0x2, Array.Empty<Operand>()),                            // exit
+        }, new[] { "A", "B" });
+
+    [Fact]
+    public void Default_PlowsPastWait_EmitsBothPages()
+    {
+        var t = Table();
+        var host = new RecordingHost();
+        var vm = new VirtualMachine(TwoPage(t), t, host);   // default VmOptions => HaltAtWaitForInput=false
+        vm.Run();
+        Assert.Equal(2, host.Lines.Count);          // both pages emitted (plow)
+        Assert.Equal("exit", vm.HaltReason);
+    }
+
+    [Fact]
+    public void HaltAtWait_StopsAtFirstWait_EmitsOnlyFirstPage()
+    {
+        var t = Table();
+        var host = new RecordingHost();
+        var vm = new VirtualMachine(TwoPage(t), t, host, new VmOptions(HaltAtWaitForInput: true));
+        vm.Run();
+        Assert.Single(host.Lines);                  // only page A ran; halted at the wait
+        Assert.Equal("wait-for-input", vm.HaltReason);
+    }
+}