feat(mini-c): connectivity probe core with unit tests

Adds the pure/core probe logic for the standalone C client (hbc_mini.c):
gateway text parsers for /proc/net/route, /proc/net/ipv6_route and BSD
`route -n get`, a ping-output RTT parser, and the probe_step state machine
(probe_t/probe_fam_t/probe_ops_t) plus probe_flags and probe_metric_json.
config_t gains probe_enabled/probe_interval/probe_v4/probe_v6.

Temporary stub functions (sys_local_address, sys_ping, sys_gateway,
mono_ts) stand in for real syscalls until Task 4 wires them into main().

New scripts/c/test_probe.c test harness (#include's hbc_mini.c with main
renamed out of the way) plus a `make test` target; scripts/c/.gitignore
now ignores the test_probe binary.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uc9V1v7iXruubpoLiSE24M

scripts/c/Makefile

@@ -7,7 +7,7 @@ SRC     = hbc_mini.c
 # FreeBSD/NetBSD keep zlib in base; no extra flags needed.
 # On some NetBSD installs pthreads may need -lpthread from pkgsrc.
 
-.PHONY: all clean debug
+.PHONY: all clean debug test
 
 all: $(TARGET)
 
@@ -17,5 +17,11 @@ $(TARGET): $(SRC)
 debug: $(SRC)
 	$(CC) -g -fsanitize=address,undefined -o $(TARGET)_dbg $< $(LDFLAGS)
 
+test: test_probe
+	./test_probe
+
+test_probe: test_probe.c $(SRC)
+	$(CC) $(CFLAGS) -o $@ test_probe.c $(LDFLAGS)
+
 clean:
-	rm -f $(TARGET) $(TARGET)_dbg
+	rm -f $(TARGET) $(TARGET)_dbg test_probe