test: run test/lang @test corpus via the test-lang make target

The four test/lang/*_test.ww in-language @test files had no make target
running them — dead coverage that could rot silently. Add a standalone
test-lang target running each through the cstage `ww test` subcommand
(one package per invocation; set -e gives the gate teeth). Imports
resolve via the driver's self_dir/../../lib fallback, so no lib env.

Kept out of make test/test-unit/test-commit: that wiring is fold 6 and
is gated on the T1 behavior corpus reaching parity before byte-id is
demoted to pre-push.

Makefile

@@ -3675,6 +3675,17 @@ RUNWW_CASES = $(wildcard test/wcc/data/*/case.ww)
 test-run: all
 	@BIN=$(BIN) $(BIN)/ww run test/runww.ww $(RUNWW_CASES)
 
+# ---- test-lang: in-language @test behavior corpus (fold 2) -------------
+# Runs each test/lang/*_test.ww through the cstage `ww test` subcommand.
+# Each file is its own package, so one file per invocation. A failing
+# @test exits non-zero and fails the target. Imports (rt/strings) resolve
+# via the driver's self_dir/../../lib fallback, so no lib env is needed.
+# NOT yet wired into `make test` (fold 6, gated on the T1 corpus).
+test-lang: all
+	@set -e; for f in $(wildcard test/lang/*_test.ww); do \
+		echo "ww test $$f"; $(BIN)/ww test $$f; \
+	done; echo "test-lang: all ok"
+
 # ---- test-commit: per-commit default (task #10 T1) ---------------------
 # Full suite, but content-key-SKIPS the expensive wwstage byte-id /
 # self-compile gates (950/989_lib_byteid/990-997) when their input set is
@@ -3865,4 +3876,4 @@ nocc:
 	@echo
 	@echo "NOCC OK: $(STAGE0)/* reproduces itself from source. cc not invoked."
 
-.PHONY: all cstage wwstage test test-unit test-commit test-run smoke sizelint peellint install clean bootstrap nocc bootstrap-snapshot
+.PHONY: all cstage wwstage test test-unit test-commit test-run test-lang smoke sizelint peellint install clean bootstrap nocc bootstrap-snapshot