Assert the PSI wiring the ParserDefinition unblocked

JaiFileTypeTest could previously only check the virtual file's type, because
without a ParserDefinition the PSI file fell back to plain text. It now asserts
the PSI file is a JaiFile in the Jai language and that JaiParserDefinition is the
registered definition - the wiring every PSI-dependent feature needs.

Also records the in-memory-fixture flake ('Cannot create child file at /src') as
infrastructure, with the note that JaiCorpusParserTest sidesteps it by using
PsiFileFactory directly.

AGENTS.md

@@ -60,7 +60,7 @@ Last verified state (all green, `./jaigradle check` and `verifyPlugin` too):
 
 ```text
 dev.hgh.HarnessSmokeTest                    tests=2
-dev.hgh.jai.JaiFileTypeTest                 tests=3
+dev.hgh.jai.JaiFileTypeTest                 tests=4
 dev.hgh.jai.editor.JaiEditorSupportTest     tests=5
 dev.hgh.jai.highlighting.*                  tests=8
 dev.hgh.jai.lexer.JaiCorpusLexerTest        tests=2   <- the Tier 0 gate
@@ -68,7 +68,7 @@ dev.hgh.jai.lexer.JaiLexerTest              tests=13
 dev.hgh.jai.parser.JaiCorpusParserTest      tests=1   <- the Tier 3 gate
 dev.hgh.jai.parser.JaiParserGoldenTest      tests=5   <- Tier 2 golden trees
 dev.hgh.jai.parser.DebugParseTest           tests=2   <- scratch harness, inert
--> total 41, failures+errors 0
+-> total 42, failures+errors 0
 ```
 
 The corpus gates report what they actually did; check both lines are still there:
@@ -268,6 +268,11 @@ The fast loop for grammar work:
   ./jaitest
   ```
 
+- **`Cannot create child file 'x' at /src` is a flake.** The fixture's in-memory
+  filesystem occasionally refuses `myFixture.configureByText`. It is not a real
+  failure — re-run. `JaiCorpusParserTest` avoids it entirely by going through
+  `PsiFileFactory.createFileFromText` instead of the fixture, which also makes
+  the 714-file sweep much faster.
 - **`timeout` does not exist on this macOS box.** Do not use it in scripts.
 - **Noisy test stderr.** Fixture runs log Vue/JS `PluginException`s from bundled
   plugins. Tests pass regardless, but real failures can be buried — check the

src/test/kotlin/dev/hgh/jai/JaiFileTypeTest.kt

@@ -1,7 +1,10 @@
 package dev.hgh.jai
 
+import com.intellij.lang.LanguageParserDefinitions
 import com.intellij.openapi.fileTypes.FileTypeManager
 import com.intellij.testFramework.fixtures.BasePlatformTestCase
+import dev.hgh.jai.psi.JaiFile
+import dev.hgh.jai.psi.JaiParserDefinition
 
 /**
  * Verifies plugin.xml wiring: the `.jai` extension resolves to our file type and
@@ -24,12 +27,24 @@ class JaiFileTypeTest : BasePlatformTestCase() {
     }
 
     /**
-     * The *virtual* file already resolves to our type. The PSI file's language stays
-     * plain text until a ParserDefinition is registered (phase 3) — asserting that here
-     * would be asserting the platform's fallback, not our wiring.
+     * Before phase 3 this could only assert the *virtual* file's type, because
+     * without a `ParserDefinition` the PSI file fell back to plain text. Now that
+     * `JaiParserDefinition` is registered, the PSI file itself is a [JaiFile] — which
+     * is what every PSI-dependent feature (structure view, folding, references)
+     * relies on.
      */
-    fun testVirtualFileUsesJaiFileType() {
+    fun testJaiFileProducesJaiPsi() {
         val file = myFixture.configureByText("sample.jai", "main :: () { }")
         assertEquals(JaiFileType, file.virtualFile.fileType)
+        assertEquals(JaiLanguage, file.language)
+        assertTrue("expected a JaiFile, got ${file.javaClass.name}", file is JaiFile)
+    }
+
+    fun testParserDefinitionIsRegisteredForJai() {
+        val definition = LanguageParserDefinitions.INSTANCE.forLanguage(JaiLanguage)
+        assertTrue(
+            "plugin.xml must register our parser definition, got $definition",
+            definition is JaiParserDefinition,
+        )
     }
 }