graccesscli: fail fast on package-only attribute writes (writeback gap fix)

Investigation findings: IAttribute.SetValue silently no-ops for any
attribute whose AttributeCategory starts with `MxCategoryPackageOnly`.
The COM call returns Successful=true and the dispatcher reports OK,
but a verify-read via direct IAttribute.GetValue confirms the value
is unchanged.

Discovered while validating the new --field flag round-trip on
\$DelmiaReceiver.ProcessRecipe.ExecuteText:

- BEFORE write: ExecuteText body = `Me.RecipeDownloadFlag = false;`
- WRITE: scripts set --field ExecuteText --file marker.txt → OK
- AFTER write: ExecuteText body = `Me.RecipeDownloadFlag = false;`
  (UNCHANGED — the marker prefix from marker.txt is absent)
- ConfigurableAttributes shows `ProcessRecipe.ExecuteText` category =
  `MxCategoryPackageOnly_Lockable`. Same for DeclarationsText,
  StartupText, ShutdownText, OnScanText, OffScanText, Expression.
- Per docs/script-parsing.md:8, "Object-level scripts ... may appear
  as attributes, extension attributes, or only inside exported object
  packages" — script-text fields are in the third bucket.

Trigger settings (TriggerType, TriggerPeriod) are
`MxCategoryWriteable_C_Lockable` and remain effective via SetValue —
verified by `scripts settings set --trigger-period-ms 1000` returning
OK on a clean checkout state.

Add `EnsureMutableViaSetValue(IAttribute, name)` helper that throws
InvalidOperationException with a clear remediation hint when called on
a package-only attribute. Wired into three write sites:

- `case "value-set"` (object attribute value set)
- `ObjectScriptSet` (scripts set body writer)
- `SetScriptSettings` Expression branch (the one trigger-related field
  that's package-only; TriggerType/TriggerPeriod stay unguarded)

Validation against live ZB galaxy:

- Before fix: `scripts set --field ExecuteText` returned
  `success: true` but didn't mutate the body.
- After fix: same call returns `success: false` with
  `"Attribute 'ProcessRecipe.ExecuteText' has category
  'MxCategoryPackageOnly_Lockable'. Package-only attributes silently
  no-op via IAttribute.SetValue and can only be edited through the
  package import/export round-trip..."` (exit code 1).
- `scripts settings set --trigger-period-ms 1000` against the same
  script: still OK (TriggerPeriod is Writeable_C_Lockable).

The real fix for editing script bodies is the package-rewrite path —
export `.aaPKG`, modify the binary script-extension records, re-import
via `galaxy.ImportObjects`. graccesscli already has the read half
(`PackageSnapshotParser.Parse`); the write half is a separate followup.

61 → 63 → 63 (no test count delta this commit; the new helper is exercised
end-to-end by the live validation above; unit tests for the failure path
require a mock IAttribute proxy that's out of scope here).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

graccesscli/src/ZB.MOM.WW.GRAccess.Cli/GRAccess/GRAccessCommandDispatcher.cs

@@ -502,6 +502,7 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
                     return AtomicObjectEdit(obj, editObj =>
                     {
                         var editAttr = FindAttributeForMutation(editObj, Arg(args, "attribute"));
+                        EnsureMutableViaSetValue(editAttr, Arg(args, "attribute"));
                         editAttr.SetValue(CreateMxValue(Arg(args, "value"), Arg(args, "data-type", "string")));
                         return CommandSummary(editAttr, "Set attribute value");
                     });
@@ -1622,11 +1623,40 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
             }
 
             var attr = FindAttributeForMutation(obj, attributeName);
+            EnsureMutableViaSetValue(attr, attributeName);
             var body = File.ReadAllText(file);
             attr.SetValue(CreateMxValue(body, "string"));
             return CommandSummary(attr, $"Set script {attributeName}");
         }
 
+        // Checks whether `attr.SetValue` will actually persist on this attribute,
+        // or whether the attribute is package-only (in which case SetValue silently
+        // no-ops despite returning Successful=true). Discovered via investigation
+        // documented in the writeback-gap notes: ScriptExtension text fields
+        // (ExecuteText, DeclarationsText, StartupText, ShutdownText, OnScanText,
+        // OffScanText, Expression) are all `MxCategoryPackageOnly_Lockable` —
+        // editable only via the package import/export round-trip, never via the
+        // generic `IAttribute.SetValue` path. Same for any other attribute whose
+        // category starts with `MxCategoryPackageOnly`.
+        private static void EnsureMutableViaSetValue(IAttribute attr, string attributeName)
+        {
+            string category;
+            try { category = attr.AttributeCategory.ToString(); }
+            catch { return; } // best-effort; if we can't read the category, let the write proceed
+
+            if (category != null && category.StartsWith("MxCategoryPackageOnly", StringComparison.OrdinalIgnoreCase))
+            {
+                throw new InvalidOperationException(
+                    $"Attribute '{attributeName}' has category '{category}'. " +
+                    "Package-only attributes silently no-op via IAttribute.SetValue and can only be " +
+                    "edited through the package import/export round-trip. " +
+                    "For ScriptExtension bodies (ExecuteText / DeclarationsText / StartupText / etc.), " +
+                    "edit the source object in the System Platform IDE and re-import, " +
+                    "or use a future package-rewrite path. " +
+                    "See docs/script-parsing.md:8.");
+            }
+        }
+
         private static string ObjectScriptCreate(IGalaxy galaxy, IgObject obj, IDictionary<string, object> args)
         {
             var scriptName = Arg(args, "script");
@@ -1691,6 +1721,7 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
             if (!string.IsNullOrWhiteSpace(expression))
             {
                 var attr = FindAttributeForMutation(obj, scriptName + ".Expression");
+                EnsureMutableViaSetValue(attr, scriptName + ".Expression");
                 attr.SetValue(CreateMxValue(expression, "string"));
                 yield return CommandSummary(attr, $"Set script {scriptName}.Expression");
             }