fix(roadflare-rider): emit authoritative fiat fare in non-Bitcoin RoadFlare offers

[variablefate]

Summary

ADR-0008 added `fare_fiat_amount` and `fare_fiat_currency` to Kind 3173 for non-Cashu / non-Lightning rides. The receive side was wired in drivestr by #61 (`FiatFare` data class, full UI integration, process-death persistence). The Android rider-app "send" side was wired in a follow-up — `RiderViewModel` passes `fareFiatAmount` + `fareFiatCurrency` through 15+ call sites.

The remaining gap is roadflare-rider (the Android RoadFlare-only rider app): it has zero references to either field name (`FiatFare`, `fiatFare`, `fareFiatAmount`, `fareFiatCurrency`). Non-Bitcoin RoadFlare offers (cash, Zelle, PayPal, etc.) sent from this app omit the authoritative fiat fields entirely, forcing iOS and drivestr receivers to fall back to sats→USD conversion and re-introducing the ~$1 drift ADR-0008 was meant to eliminate.

Verification

```
grep -rn "FiatFare\|fiatFare\|fareFiatAmount\|fareFiatCurrency" roadflare-rider/ --include="*.kt"
→ zero matches
```

For comparison, drivestr has full coverage (DriverModeScreen, DriverModeScreenComponents, DriverViewModel, RoadflareNotificationFormattingTest) and rider-app has 15+ send-side call sites in RiderViewModel.

Scope

• Compute `fareFiatAmount` + `fareFiatCurrency` in roadflare-rider's offer flow when the selected payment method is non-Bitcoin (matches rider-app's existing logic — pickup→dropoff route × per-mile rate + base fare).
• Pass the fields through the `RideOfferSpec.RoadFlare` send path so they reach `RideOfferEvent.create()` JSON.
• Boost path: drop both fields per CLAUDE.md "Boost behavior" / ADR-0008 (driver falls back to sats→USD for the boosted offer only).
• Cashu / Lightning offers: do NOT emit the fields per the "both-or-neither" rule scoped to fiat-rail offers.

Acceptance criteria

• roadflare-rider sends a Cash / Zelle / PayPal RoadFlare offer → on-the-wire Kind 3173 contains both `fare_fiat_amount` and `fare_fiat_currency`.
• Boost path drops both fields; receiving driver falls back to sats→USD for the boosted offer only.
• Cashu / Lightning offers from roadflare-rider remain unchanged (no new fields).
• Regression test extends `RoadflareWiringTest` (or a new test class) to pin serialization for non-Bitcoin offers.
• Manual cross-platform: roadflare-rider → drivestr displays the rider's exact fiat amount; roadflare-rider → roadflare-ios same.

Related

• ADR-0008 — Authoritative fiat fare on Kind 3173 (the protocol contract).
• fix(drivestr): display authoritative fiat fare from iOS offer events #61 — drivestr receive-side wiring (already shipped). Reference for naming, data class, and parse semantics.
• rider-app send-side wiring (already shipped). `RiderViewModel` is the implementation reference.
• refactor(roadflare): decouple ride-on-demand from key state + actually-filter mute #82 — Mute + stale-key refactor; fare-display correctness becomes more visible once stale-key requests are allowed through.
• fix(rider-app): wire per-driver-fare fallback to also cover stale/missing key case #83 — Investigation that fare calc never depends on driver location.