Reduce network startup and reconnect delays

[cpunt]

Network interfaces take too long to become usable and show online after boot or modem reconnection. Remove avoidable delays between modem readiness, interface publication, network activation and UI status updates.

Evidence

Observed on BigBox SS with firmware bigbox-ss-v0.11.2-2026-09-09T103923-v22.03.7, including the HAL late-reply fix from #331:

• GSM discovers and publishes the modem's network interface during its metrics poll. That loop waits before its first iteration; BigBox SS configures a 60-second interval. See interface discovery, initial polling delay, and target configuration.
• During SSH observation on 9 September 2026, primary was detected at 14:47:39 UTC, but its wwan0 binding was not published until 14:48:41 UTC. ModemManager had already reported the modem connected at 14:47:46 UTC.
• Separate modem arrivals trigger successive structural network applies. In the initial logs, secondary arrived at 14:38:22 UTC, primary superseded its apply at 14:38:24 UTC, and the replacement took 27.1 seconds.
• Structural applies reload networking and restart dnsmasq, firewall and mwan3. Logs show existing WAN and cellular connections temporarily disconnecting during these applies.
• Publishing GSM connection state also performs synchronous diagnostic reads, which can delay state delivery when modem requests are slow. Their contribution to the observed delay needs measurement.

The 60-second dependency predates #331. Keep that deadlock fix. Manual SIM swaps may explain the observed modem disappearances; this issue focuses on startup and recovery latency, not diagnosing those disappearances.

Requested improvements

• Discover and publish the network interface promptly during modem initialization and reconnection, independently of periodic metrics.
• Publish essential connection state without waiting for firmware, band, operator or signal enrichment; update diagnostics separately.
• Coalesce closely spaced interface changes and avoid unnecessary service restarts or repeated activation work.
• Measure elapsed time from modem detection through APN connection, interface publication, network activation and UI readiness using monotonic timestamps. The initial boot logs include a wall-clock correction, so wall-clock timestamps alone are insufficient across that transition.

Acceptance and validation

• Healthy interfaces become available without waiting for a metrics interval or optional diagnostic reads.
• Test cold boot, devicecode restart, SIM reconnection, and both modems arriving close together.
• Slow diagnostics on one modem do not delay readiness reporting for the other.
• Verify actual network readiness alongside UI status, and record before/after timings.
• Add focused GSM/network regressions and run the Lua suite. Preserve existing public interfaces and the Prevent late HAL replies from blocking control workers #331 deadlock fix.
• Check the shared behavior for bigbox-ss, bigbox-v1-cm, and bigbox-v1-cm-2; identify which targets receive hardware validation.

Related: #330 tracks the separate case where mwan3 is already online but the UI remains offline. Coordinate the fixes while distinguishing actual connection latency from stale UI status.