Phase 1 gate: Tier 0 corpus lexer invariants over the whole Jai distribution

Lexes all 714 .jai files (17.1M chars, 3.0M tokens) and asserts round-trip
tiling, strict offset progress, and zero BAD_CHARACTER. Green on first run.

jaitest: use cleanTest and retry once when a concurrent Gradle run clobbers
build/test-results (java.io.EOFException with no XML).

jaitest

@@ -4,19 +4,38 @@
 # A green `test` task does not prove tests ran (see AGENTS.md), so this always
 # prints tests/failures/errors per suite and dumps failure messages.
 #
+# Two concurrent Gradle invocations (e.g. an editor/agent running tests in the
+# background at the same time) fight over build/test-results and the loser dies
+# with `java.io.EOFException` or a missing in-progress-results bin. That is an
+# infrastructure failure, not a test failure, so it is retried once.
+#
 # Usage: ./jaitest [gradle args...]      e.g. ./jaitest --tests '*Lexer*'
 set -uo pipefail
 
 cd "$(dirname "$0")" || exit 1
 
-rm -rf build/test-results/test
-./jaigradle test "$@" 2>&1 | grep -vE '^\[[0-9.]+s\]\[warning\]\[cds\]'
-gradle_status=${PIPESTATUS[0]}
+run_gradle() {
+    rm -rf build/reports/tests/test
+    ./jaigradle cleanTest test "$@" 2>&1 | grep -vE '^\[[0-9.]+s\]\[warning\]\[cds\]'
+    return "${PIPESTATUS[0]}"
+}
+
+run_gradle "$@"
+gradle_status=$?
+
+shopt -s nullglob
+xml=(build/test-results/test/*.xml)
+if [ "$gradle_status" -ne 0 ] && [ ${#xml[@]} -eq 0 ]; then
+    echo
+    echo "!!! no test XML and a failed build — retrying once (concurrent Gradle run?)"
+    sleep 3
+    run_gradle "$@"
+    gradle_status=$?
+    xml=(build/test-results/test/*.xml)
+fi
 
 echo
 echo "=== JUnit XML ==="
-shopt -s nullglob
-xml=(build/test-results/test/*.xml)
 if [ ${#xml[@]} -eq 0 ]; then
     echo "NO TEST XML PRODUCED — tests did not run."
     exit 1