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