Refactor rename: AssertionError: Diff not implemented: SNIPPET

### Apache NetBeans version

Apache NetBeans 30

### What happened

With
```
interface RefactorSnippet {
/**
* {@snippet :
* update();
* }
*/
default void update() {
}
}
```
1. Place cursor on `update` on line `default void update() {`
2. Do refactor rename
3. check box `Apply rename on comments`
4. Click refactor

Observe `AssertionError`

### Language / Project Type / NetBeans Component

java refactor

### How to reproduce

See OP

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Linux harmony 7.0.9-76070009-generic #202605191504~1779464622~22.04~ef31795 SMP PREEMPT_DYNAMIC Fri M x86_64 x86_64 x86_64 GNU/Linux

### JDK

openjdk 26 2026-03-17 OpenJDK Runtime Environment (build 26+35-2893) OpenJDK 64-Bit Server VM (build 26+35-2893, mixed mode, sharing)

### Apache NetBeans packaging

Apache NetBeans binary zip

### Anything else

```
INFO [org.netbeans.modules.refactoring.spi.impl.ParametersPanel]: org.netbeans.modules.refactoring.api.AbstractRefactoring$ProgressL@5b1ea785 called start multiple times
INFO [org.netbeans.modules.refactoring.spi.impl.ParametersPanel]: org.netbeans.modules.refactoring.api.AbstractRefactoring$ProgressL@5b1ea785 called start multiple times
SEVERE [org.openide.util.Exceptions]
java.lang.AssertionError: Diff not implemented: SNIPPET com.sun.tools.javac.tree.DCTree$DCSnippet
at org.netbeans.modules.java.source.save.CasualDiff.diffDocTree(CasualDiff.java:4891)
at org.netbeans.modules.java.source.save.CasualDiff.diffList(CasualDiff.java:5310)
at org.netbeans.modules.java.source.save.CasualDiff.diffDocComment(CasualDiff.java:4924)
at org.netbeans.modules.java.source.save.CasualDiff.diffDocTree(CasualDiff.java:4779)
at org.netbeans.modules.java.source.save.CasualDiff.diffCommentLists(CasualDiff.java:4674)
at org.netbeans.modules.java.source.save.CasualDiff.diffPrecedingComments(CasualDiff.java:4565)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl0(CasualDiff.java:5664)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl(CasualDiff.java:5603)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5530)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5501)
at org.netbeans.modules.java.source.save.CasualDiff.diffList(CasualDiff.java:4314)
at org.netbeans.modules.java.source.save.CasualDiff.diffClassDef(CasualDiff.java:1204)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl0(CasualDiff.java:5714)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl(CasualDiff.java:5603)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5530)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5501)
at org.netbeans.modules.java.source.save.CasualDiff.diffList(CasualDiff.java:4314)
at org.netbeans.modules.java.source.save.CasualDiff.diffTopLevel(CasualDiff.java:602)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl0(CasualDiff.java:5684)
at org.netbeans.modules.java.source.save.CasualDiff.diffTreeImpl(CasualDiff.java:5603)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5530)
at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:5513)
at org.netbeans.modules.java.source.save.CasualDiff.diff(CasualDiff.java:358)
at org.netbeans.api.java.source.WorkingCopy.processCurrentCompilationUnit(WorkingCopy.java:905)
at org.netbeans.api.java.source.WorkingCopy.getChanges(WorkingCopy.java:1320)
at org.netbeans.api.java.source.JavaSource$1.run(JavaSource.java:672)
at org.netbeans.api.java.source.JavaSource$1.run(JavaSource.java:661)
at org.netbeans.api.java.source.JavaSource$MultiTask.run(JavaSource.java:504)
at org.netbeans.modules.parsing.impl.TaskProcessor.callUserTask(TaskProcessor.java:586)
at org.netbeans.modules.parsing.api.ParserManager$MultiUserTaskAction.run(ParserManager.java:235)
at org.netbeans.modules.parsing.api.ParserManager$MultiUserTaskAction.run(ParserManager.java:205)
at org.netbeans.modules.parsing.impl.TaskProcessor$2.call(TaskProcessor.java:181)
at org.netbeans.modules.parsing.impl.TaskProcessor$2.call(TaskProcessor.java:178)
at org.netbeans.modules.masterfs.filebasedfs.utils.FileChangedManager.priorityIO(FileChangedManager.java:153)
at org.netbeans.modules.masterfs.providers.ProvidedExtensions.priorityIO(ProvidedExtensions.java:341)
at org.netbeans.modules.parsing.nb.DataObjectEnvFactory.runPriorityIO(DataObjectEnvFactory.java:118)
at org.netbeans.modules.parsing.impl.Utilities.runPriorityIO(Utilities.java:67)
at org.netbeans.modules.parsing.impl.TaskProcessor.runUserTask(TaskProcessor.java:178)
at org.netbeans.modules.parsing.api.ParserManager.parse(ParserManager.java:85)
at org.netbeans.api.java.source.JavaSource.runUserActionTaskImpl(JavaSource.java:454)
at org.netbeans.api.java.source.JavaSource.runUserActionTask(JavaSource.java:425)
at org.netbeans.api.java.source.JavaSource.runModificationTask(JavaSource.java:680)
at org.netbeans.modules.refactoring.java.spi.JavaRefactoringPlugin.processFiles(JavaRefactoringPlugin.java:325)
at org.netbeans.modules.refactoring.java.spi.JavaRefactoringPlugin.processFiles(JavaRefactoringPlugin.java:264)
at org.netbeans.modules.refactoring.java.spi.JavaRefactoringPlugin.processFiles(JavaRefactoringPlugin.java:246)
at org.netbeans.modules.refactoring.java.spi.JavaRefactoringPlugin.createAndAddElements(JavaRefactoringPlugin.java:340)
at org.netbeans.modules.refactoring.java.spi.JavaRefactoringPlugin.createAndAddElements(JavaRefactoringPlugin.java:356)
at org.netbeans.modules.refactoring.java.plugins.RenameRefactoringPlugin.prepare(RenameRefactoringPlugin.java:613)
[catch] at org.netbeans.modules.refactoring.api.AbstractRefactoring.pluginsPrepare2(AbstractRefactoring.java:417)
at org.netbeans.modules.refactoring.api.AbstractRefactoring.pluginsPrepare(AbstractRefactoring.java:401)
at org.netbeans.modules.refactoring.api.AbstractRefactoring.prepare(AbstractRefactoring.java:212)
at org.netbeans.modules.refactoring.spi.impl.ParametersPanel$Prepare.run(ParametersPanel.java:1063)
at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1370)
at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
at org.openide.util.lookup.Lookups.executeWith(Lookups.java:287)
at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:1981)
```

### Are you willing to submit a pull request?

No

Start in java.source.save.CasualDiff at diffDocTree around line 4891 and reproduce the rename with “Apply rename on comments” using the snippet shown in the issue. Trace the SNIPPET handling through diffDocComment and verify that the refactoring completes without AssertionError while preserving the intended comment update.

Written by the indexing model from the issue text.