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/support.h

@@ -0,0 +1,53 @@
+/*
+ * support.h — test harness API for off-target firmware unit tests.
+ *
+ * Provides the mock environment (serial ring, command buffers, output capture)
+ * plus a tiny assertion framework. The firmware sources under test are compiled
+ * unmodified against this; only the hardware edges are mocked here.
+ */
+#ifndef TEST_SUPPORT_H
+#define TEST_SUPPORT_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/* Distinct-named string helpers so host code needn't include <string.h>, which
+ * would clash with the firmware's own memset/strlen/strcpy prototypes in
+ * rtl837x_common.h (different 8051 signatures). */
+static inline void  t_memset(void *d, int c, size_t n) {
+    unsigned char *p = d; while (n--) *p++ = (unsigned char)c;
+}
+static inline size_t t_strlen(const char *s) { const char *p = s; while (*p) p++; return (size_t)(p - s); }
+static inline int    t_strcmp(const char *a, const char *b) {
+    while (*a && *a == *b) { a++; b++; } return (unsigned char)*a - (unsigned char)*b;
+}
+
+/* ---- captured terminal output (what the firmware would send over serial) ---- */
+extern char     out_buf[8192];
+extern size_t   out_len;
+void            out_reset(void);
+
+/* ---- editor/parser environment reset ---- */
+void env_reset(void);           /* clears ring, buffers, editor state */
+
+/* ---- drive the serial input ring exactly like the RX ISR would ----
+ * Pushes bytes into the 16-byte sbuf ring, draining via cmd_edit() whenever the
+ * ring would overflow, then a final drain. Mirrors real ISR→editor handoff.
+ */
+void feed(const char *s);                 /* NUL-terminated */
+void feed_n(const char *s, size_t n);     /* explicit length (for embedded NULs) */
+
+/* ---- watchdog: arm before calling into code that could spin (C4 hang) ---- */
+void watchdog_arm(unsigned seconds, const char *what);
+void watchdog_disarm(void);
+
+/* ---- minimal test framework ---- */
+extern int tests_run, tests_failed;
+#define CHECK(cond, msg) do {                                            \
+        tests_run++;                                                     \
+        if (!(cond)) { tests_failed++;                                   \
+            printf("  FAIL: %s  (%s:%d)\n", (msg), __FILE__, __LINE__);  \
+        } else { printf("  ok:   %s\n", (msg)); }                        \
+    } while (0)
+
+#endif /* TEST_SUPPORT_H */