qtdeclarative: durably regenerate qlalr grammar on incremental rebuild

A relibc ABI change invalidates qtdeclarative's fingerprint and triggers an
incremental rebuild that keeps the 1-line empty qqmljsgrammar_p.h stub (it
survives the recipe's CMakeCache/CMakeFiles removal), so ninja skips qlalr
regeneration and every consumer of qqmljslexer_p.h fails with 'expected
class-name' / 'QQmlJS::Parser incomplete type'. Delete the stale qlalr outputs
(qqmljsgrammar_p.h + qqmljsgrammar.cpp) before configure so qt_process_qlalr
regenerates them. Pattern (j): stale codegen after a toolchain rebuild — now
durable instead of needing a manual clean rebuild each time.

Validated: qtdeclarative built (libQt6Qml/libQt6Quick) in the full cascade build.

local/recipes/qt/qtdeclarative/recipe.toml

@@ -148,6 +148,18 @@ fi
 rm -f CMakeCache.txt
 rm -rf CMakeFiles
 
+# Remove stale qlalr-generated grammar. The QML lexer/parser grammar
+# (qqmljsgrammar_p.h + qqmljsgrammar.cpp) is generated by qt_process_qlalr from
+# src/qml/parser/qqmljs.g; the source ships none. An incremental rebuild (e.g.
+# after a relibc ABI change invalidates the fingerprint) can leave a 1-line empty
+# stub that survives CMakeCache/CMakeFiles removal, so ninja considers it up to
+# date and skips regeneration — every consumer of qqmljslexer_p.h then fails with
+# "expected class-name" / "QQmlJS::Parser has incomplete type". Deleting the
+# stale outputs forces qt_process_qlalr to regenerate them (missing output ->
+# rule re-runs). This is pattern (j): stale codegen after a toolchain rebuild.
+find . -name 'qqmljsgrammar_p.h' -delete 2>/dev/null || true
+find . -name 'qqmljsgrammar.cpp' -delete 2>/dev/null || true
+
 redbear_qt_link_sysroot_dirs "${COOKBOOK_SYSROOT}" plugins mkspecs metatypes modules qml
 
 # Patch masm/CheckedArithmetic.h: add missing ArithmeticOperations<unsigned, long>