test: add host-side unit-test harness, covering cmd_editor

Compile selected firmware translation units unmodified under host gcc with
AddressSanitizer and UBSan, force-including a shim header that erases the
SDCC 8051 keywords, and link them against a mocked serial ring and console.
A first suite covers the CLI line editor: line entry, backspace editing,
and both long-input overflow cases (length cap and ring-pointer stall).

The only firmware-source change is one #ifndef RTLP_HOST_TEST block in
rtl837x_common.h grouping the libc-named prototypes (sleep/memcpy/memset/
strlen/strcpy/strcmp) so host builds take them from the C library instead;
on-target builds are unaffected.

Run with: make -C test

test/Makefile

@@ -0,0 +1,45 @@
+# Host unit-test harness for RTLPlayground firmware logic.
+#
+# Compiles selected firmware translation units with host gcc + AddressSanitizer,
+# force-including sdcc_shim.h to erase 8051 keywords, and links them against
+# mocked hardware edges (support.c). No SDCC, no flashing, no hardware.
+#
+#   make        build + run all tests
+#   make run    run tests
+#   make clean
+
+CC       = gcc
+SRCROOT  = ..
+# -include shim first so __xdata/__code/__banked vanish before any firmware header.
+CFLAGS   = -std=gnu11 -g -O1 -Wall -Wextra -Wno-unused-parameter -Wno-unknown-pragmas \
+           -Wno-builtin-declaration-mismatch -DRTLP_HOST_TEST \
+           -fsanitize=address,undefined -fno-omit-frame-pointer \
+           -include sdcc_shim.h -I. -I$(SRCROOT) -I$(SRCROOT)/httpd -I$(SRCROOT)/uip
+LDFLAGS  = -fsanitize=address,undefined
+
+BUILDDIR = build
+
+# One binary per subsystem under test.
+TESTS = $(BUILDDIR)/test_cmd_editor
+
+all: run
+
+$(BUILDDIR):
+	mkdir -p $(BUILDDIR)
+
+# cmd_editor.c is compiled UNMODIFIED from the firmware tree.
+$(BUILDDIR)/test_cmd_editor: test_cmd_editor.c support.c $(SRCROOT)/cmd_editor.c | $(BUILDDIR)
+	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS)
+
+run: $(TESTS)
+	@fail=0; for t in $(TESTS); do \
+		echo "=== $$t ==="; \
+		$$t || fail=1; \
+		echo; \
+	done; \
+	if [ $$fail -ne 0 ]; then echo "TESTS FAILED"; exit 1; else echo "ALL TESTS PASSED"; fi
+
+clean:
+	rm -rf $(BUILDDIR)
+
+.PHONY: all run clean