test: skip the live Frankfurter fetch so FX moves can't shift assertions

Mirrors the pricing guard: CODEBURN_FX_NO_FETCH short-circuits getExchangeRate
after the cache read and before the fetch, returning the same USD-equivalent
rate an unreachable network already yields. Tests that seed their own
exchange-rate.json (serve-stdio's EUR case) keep working unchanged.

src/currency.ts

@@ -114,6 +114,11 @@ async function getExchangeRate(code: string): Promise<number> {
   const cached = await loadCachedRate(code)
   if (cached) return cached
 
+  // Test-only escape hatch, set for the whole suite in
+  // tests/setup/env-isolation.ts: skip the live Frankfurter fetch so a real FX
+  // move can't shift assertions. Same fallback an unreachable network gets.
+  if (process.env['CODEBURN_FX_NO_FETCH']) return 1
+
   let rate: number
   try {
     rate = await fetchRate(code)

tests/setup/env-isolation.ts

@@ -110,6 +110,9 @@ function applyIsolation(): void {
   // fetches the live upstream table, so a mid-week reprice by a provider turns
   // pricing assertions red with no local change.
   process.env['CODEBURN_PRICING_SNAPSHOT_ONLY'] = '1'
+  // Same for FX: skip the live Frankfurter fetch and fall back to the
+  // USD-equivalent rate unless a test seeds its own exchange-rate.json cache.
+  process.env['CODEBURN_FX_NO_FETCH'] = '1'
   // Pin the timezone so date grouping is deterministic regardless of the dev's
   // shell TZ. Clearing it is not enough (Node falls back to the OS zone); a
   // non-UTC TZ would otherwise shift day buckets versus a clean CI runner. A