fix(scriptanalysis): compile gate reads diagnostics only and never caches a faulted verdict (review 2026-10-21 04/M1, 04/C1)

RoslynScriptCompiler.Compile now uses Script.GetCompilation().GetDiagnostics()
with one shared, never-loading InteractiveAssemblyLoader instead of
Script.Compile(), which emitted and loaded a non-collectible assembly (and a
fresh AssemblyLoadContext) per distinct script. The catch-all that turned any
exception into a 'Compilation exception' diagnostic is gone; TryCompile and
CheckExpressionSyntax report a fault for that call only, uncached.

tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Validation/ScriptCompileVerdictCacheEvictionTests.cs

@@ -4,9 +4,8 @@ namespace ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests.Validation;
 
 /// <summary>
 /// The verdict cache's eviction policy. Overflow used to <c>Clear()</c> the
-/// whole cache, which re-opened the non-collectible <c>InteractiveAssemblyLoader</c>
-/// leak the cache exists to bound — every hot script had to be recompiled, and every
-/// recompile loads another assembly that can never be unloaded. These tests pin the
+/// whole cache, so every hot script had to be re-analysed and recompiled at once.
+/// These tests pin the
 /// replacement: eviction is segmented, so entries in active use survive it.
 ///
 /// <para>

tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Validation/ScriptCompilerTests.cs

@@ -259,4 +259,53 @@ public class ScriptCompilerTests
         Assert.Contains("'Beta'", second.Error);
         Assert.Contains("compile", second.Error, StringComparison.OrdinalIgnoreCase);
     }
+
+    // --- Review 2026-10-21 04/C1: transient faults must not become sticky verdicts ---
+
+    /// <summary>Verifies that a factory that throws leaves the cache slot empty, so the next call re-runs it.</summary>
+    [Fact]
+    public void GetOrAdd_FactoryThrows_NextCallReRunsFactory()
+    {
+        ScriptCompileVerdictCache.Clear();
+        Assert.Throws<InvalidOperationException>(() =>
+            ScriptCompileVerdictCache.GetOrAdd("SurfaceC1", "return 7;",
+                () => throw new InvalidOperationException("transient")));
+
+        var calls = 0;
+        var verdict = ScriptCompileVerdictCache.GetOrAdd("SurfaceC1", "return 7;", () =>
+        {
+            calls++;
+            return (true, null);
+        });
+
+        Assert.Equal(1, calls);
+        Assert.True(verdict.Ok);
+        Assert.Equal(0, ScriptCompileVerdictCache.Hits);
+    }
+
+    /// <summary>
+    /// Verifies that a compile gate that faults once reports a failure for that call
+    /// only — the next call with the same code re-runs the compile and succeeds.
+    /// </summary>
+    [Fact]
+    public void TryCompile_CompileFaultsOnce_FailureIsNotCached()
+    {
+        ScriptCompileVerdictCache.Clear();
+        var faults = 1;
+        var c = new ScriptCompiler(_ =>
+        {
+            if (faults-- > 0)
+                throw new System.IO.IOException("metadata read failed");
+            return [];
+        });
+        const string code = "return 8;";
+
+        var first = c.TryCompile(code, "Flaky");
+        Assert.True(first.IsFailure);
+        Assert.Contains("'Flaky'", first.Error);
+        Assert.Contains("metadata read failed", first.Error);
+        Assert.Equal(0, ScriptCompileVerdictCache.Count);   // nothing memoised
+
+        Assert.True(c.TryCompile(code, "Flaky").IsSuccess);  // pre-fix: sticky "failed to compile"
+    }
 }