Tier 2 golden trees, and #scope_file is a statement not a modifier

isDeclarationAhead used to skip *any* directive, so '#scope_file' was absorbed as
a modifier of the declaration after it. That misreports scope, since a scope
directive governs everything that follows it. Only #as, #overlay, #add_context
and #no_reset actually prefix a declaration in the corpus; the rest are
statements now, which the regenerated Directives.txt shows.

Golden fixtures cover the reference doc's gotcha list: declaration forms,
procedures (named/multiple/variadic/polymorphic/foreign returns), structs, enums,
unions, array and pointer types, the 'if x == { case }' switch form, for
modifiers, ifx, and directives including a here-string.

DebugParseTest is now inert by default and documented as the scratch loop.

generateParser wipes its output through a Delete task rather than a doFirst,
because a lambda added from a Kotlin build script captures the script object and
breaks the configuration cache ('./jaigradle check' failed to store it).

check and verifyPlugin (IU-253/261/262) both green.

build.gradle.kts

@@ -8,9 +8,11 @@ plugins {
 }
 
 // Grammar-Kit output. Committed so diffs are reviewable, never hand-edited.
+val generatedSourceDir = file("src/main/gen")
+
 sourceSets {
     main {
-        java.srcDirs("src/main/gen")
+        java.srcDirs(generatedSourceDir)
     }
 }
 
@@ -28,20 +30,25 @@ dependencies {
     }
 }
 
+// purgeOldFiles is not enough: Grammar-Kit rewrites only the files whose rule
+// changed, so a PSI interface can keep an accessor its regenerated Impl no longer has
+// (`JaiDeclaration.getArgumentList()` with no override) and the build then fails
+// inside generated code. Wiping first makes every generation self-consistent.
+//
+// This is a separate Delete task rather than a doFirst, because any lambda added to a
+// task from a Kotlin build script captures the script object, which the configuration
+// cache cannot serialize.
+val cleanGeneratedSources by tasks.registering(Delete::class) {
+    delete(generatedSourceDir)
+}
+
 tasks.generateParser {
+    dependsOn(cleanGeneratedSources)
     sourceFile.set(file("src/main/grammar/Jai.bnf"))
-    targetRootOutputDir.set(file("src/main/gen"))
+    targetRootOutputDir.set(generatedSourceDir)
     pathToParser.set("dev/hgh/jai/parser/JaiParser.java")
     pathToPsiRoot.set("dev/hgh/jai/psi")
     purgeOldFiles.set(true)
-
-    // purgeOldFiles is not enough: Grammar-Kit rewrites only the files whose rule
-    // changed, so a PSI interface can keep an accessor its regenerated Impl no longer
-    // has (`JaiDeclaration.getArgumentList()` with no override) and the build fails on
-    // generated code. Always regenerate the whole tree.
-    doFirst {
-        delete(file("src/main/gen"))
-    }
 }
 
 // Without this the compile tasks read src/main/gen with no ordering guarantee, so an