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/sdcc_shim.h

@@ -0,0 +1,40 @@
+/*
+ * sdcc_shim.h — neutralize SDCC 8051 keywords so firmware translation units
+ * compile under host gcc/clang for off-target unit testing.
+ *
+ * Force-included (gcc -include) ahead of every firmware source under test.
+ * It only erases storage/qualifier keywords and the banked-call attribute;
+ * it does NOT change any logic, so the code under test is byte-for-byte the
+ * firmware source. Hardware/register access is provided by mocks (support.c),
+ * never by this file.
+ */
+#ifndef SDCC_SHIM_H
+#define SDCC_SHIM_H
+
+#include <stdbool.h>
+
+/* Memory-space qualifiers → nothing (host has one flat address space). */
+#define __xdata
+#define __code
+#define __pdata
+#define __idata
+#define __data
+/* bool, not unsigned char: SDCC's bit types normalize any assigned value
+ * to 0/1, which _Bool reproduces exactly. */
+#define __bit         bool
+#define __sbit        bool
+
+/* NOTE: the firmware ships its own 8051 memcpy/memset/strlen/strcpy/sleep with
+ * incompatible signatures (rtl837x_common.h). Host TUs must therefore build with
+ * -fno-builtin (so gcc doesn't auto-declare the builtins) and must NOT include
+ * <string.h>/<unistd.h>. Test code uses the t_* helpers in support.h instead. */
+
+/* Function attributes → nothing (no banking / no 8051 calling conventions). */
+#define __banked
+#define __reentrant
+#define __naked
+#define __using(n)
+#define __interrupt(n)
+#define __at(addr)
+
+#endif /* SDCC_SHIM_H */