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/.gitignore

@@ -1,2 +1,3 @@
 hbc_mini
 hbc_mini_dbg
+test_probe