The single-shot connect_wifi aborted boot on any transient association or
DHCP miss — common right after a reset, when the AP still holds the stale
pre-reset association. Add a bounded retry (5 attempts, 500ms→4s backoff,
disconnect between tries) so those transient failures recover instead of
failing the boot.
Extract the logic into firmware::net (new lib target) and rewire the
wifi_tls / git_push / git_sync spikes to it, removing three duplicated
copies. main.rs reuses this when Wi-Fi lands there instead of adding a
fourth copy.
