Move IP rate-limiting, auth-failure tracking, and global connection
counting out of ssh_server.c into a dedicated module.
New API (include/ratelimit.h):
- ratelimit_init()
- ratelimit_check_ip() / ratelimit_release_ip()
- ratelimit_record_auth_failure()
- ratelimit_check_and_increment_total() / ratelimit_decrement_total()
- ratelimit_get_active_total() (replaces the direct g_total_connections
read that exec_command_stats was doing under g_conn_count_lock)
env_int() also moves up to common.{c,h} since multiple modules need it.
ssh_server.c drops from 2469 to 2200 lines. Behaviour is preserved:
the new functions are byte-for-byte the same implementations, only the
file boundary moved.
g_idle_timeout and g_access_token reads stay inline in ssh_server_init()
for now; they will follow the auth.c and input.c extractions later.
