strengthen core unit tests

Add table-driven dictionary miss and language-switch state coverage, regress stale candidates after backspace, and clear them before dictionary refresh. Keep IPC and runtime-created file checks outside the unit runner.

tests/Makefile

@@ -5,9 +5,9 @@ LIBS = -lthread -lbio
 
 PROG = unit_test
 TESTSRC = unit_test.c test_util.c str_test.c hash_test.c trie_test.c \
-	ko_test.c vi_test.c engine_test.c dict_test.c ipc_test.c
+	ko_test.c vi_test.c engine_test.c dict_test.c
 TESTOBJ = $(TESTSRC:.c=.o)
-PARENTSRC = str.c hash.c trie.c dict.c ko.c vi.c ipc.c srv.c
+PARENTSRC = str.c hash.c trie.c dict.c ko.c vi.c
 PARENTOBJ = $(PARENTSRC:%.c=unit_%.o)
 OBJS = $(TESTOBJ) $(PARENTOBJ)