Count what each provider actually serves, and name the model that answered each turn

[ParallelEntrepreneur]

#122 cherry-picked onto current main, with three trivial conflicts resolved and one compile break fixed that only appears on top of today's main. #122 is superseded by this. Closes #39.

A user wired a local DeepSeek server into Colonizer, saw Settings report it Reachable · 1 ms · 1 model, and concluded the local model was never used. Nothing was broken: the provider was pointed at subagent_model, and colonies almost never spawn subagents, so it legitimately saw almost no traffic. The gateway was the only component that knew anything, and it kept only live gauges — in-flight and queued, both zero whenever nobody is mid-request, which is almost always. "Is this thing ever used?" had no answer in the UI.

This adds the record, and makes the UI say what it knows and no more.

• Cumulative counters per provider. requests, failures, fallbacks, duration_ms, last_request_at, backed by atomics with a Timed RAII guard so a panic mid-request still counts. They survive a restart in provider-usage.json under the data directory, written back when dirty by a flush loop; a missing or corrupt file means the counters start over, never that the gateway fails to build.
• The model that answered is named per turn. The chat shows the model that did the substantive work, with +N and a tooltip when more than one was involved.
• Settings reports what it measured rather than implying traffic that never happened.

What changed on the way

Conflict	Resolution
main.rs startup tasks	Kept both spawns — version::run (the update check, merged since) and gateway::flush_loop
ChatPanel.tsx	Kept both additions; they are independent
SettingsDialog.tsx imports	Union of the two lists, so meshBroken (#128) and formatDuration (this branch) both survive

One break git could not see. Gateway::new() grew a data_dir argument on this branch, and main.rs has a test helper — added on main after this branch was cut — that still called it with none. The cherry-pick merged cleanly and then failed to compile:

error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> crates/colonizer/src/main.rs:621:22

Fixed here by passing the helper's own temp root. Worth noting because a textual merge would have produced exactly the same break with no conflict to warn anyone.

Testing

cargo test -p colonizer-harness — 180 pass (three new). npx tsc --noEmit clean, npm run build succeeds. The web suite has no npm test script yet; #132 adds one.