fix(testing): dispatch json nested-Promise guard + scoped routeMap for flight primitives (#582) - #583

Addresses two @rangojs/router/testing parity gaps from #582 (the testing-refinements split from #572). Both are cases where a testing primitive diverged from production, so a test could pass while the real router would behave differently — defeating the dogfood guarantee.

1. dispatch() json route lacked production's nested-Promise guard

Problem (consumer-observable): A json() response-route handler with a forgotten await returns a nested unresolved Promise. Production (response-route-handler.ts) serializes with a replacer that throws RESPONSE_NOT_SERIALIZABLE → a problem+json 500. dispatch() did a bare JSON.stringify, which serializes a Promise to {} — so the test went green with {"data":{}} while production 500s.

// A handler with a forgotten await:
path.json("/api/thing", () => ({ data: fetchSomething() }))  // missing await

// Before: dispatch(...) -> 200 {"data":{}}   (green, but a lie)
// After:  dispatch(...) -> 500 application/problem+json, code RESPONSE_NOT_SERIALIZABLE
//         — exactly what the real router returns.

Fix: extracted production's replacer into a shared lightweight module rsc/json-route-result.ts and routed BOTH production and dispatch through it, so they cannot drift. dispatch's existing catch already maps the thrown RouterError to the same problem+json 500.

Why its own module, not response-route-handler.ts: importing that module into dispatch dragged its heavy runtime graph (cache scope, pattern matching) which transitively reaches Vite virtual modules — breaking a plain (non-Vite) vitest import. Caught in-loop: run-in-request-context.test.ts went red with Received protocol 'virtual:'. The dedicated module depends only on errors.js.

2. renderToFlightString / renderServerTree leaked ctx.reverse to the global route map

Problem: these primitives rebuilt ctx.reverse against getGlobalRouteMap(), so a server component that calls ctx.reverse()/href() was order-dependent on whatever router registered last in the test process. renderHandler already has a scoped routeMap option; the flight primitives did not.

// After: scoped, deterministic reverse — independent of global registration order.
await renderToFlightString(<ProductLink />, {
  routeMap: { product: "/products/:id" },
});

Fix: added a routeMap?: Record<string, string> option (mirroring renderHandler), threaded through setRequestContextParams(params, routeName, routeMap?). Backward compatible — every production caller omits it and keeps getGlobalRouteMap().

Semantics

Test plan

• Unit, red-before-green: dispatch json route returning a nested Promise → problem+json 500 (not a green {}); proven red against the pre-fix bare JSON.stringify.
• RSC (dev/prod-agnostic, react-server project): renderToFlightString AND renderServerTree ctx.reverse() scoped to a routeMap resolves /scoped/products/:id; proven red (Unknown route: "product") against the empty global map without the pass-through.
• Regression caught & fixed in-loop: the virtual: import leak (run-in-request-context.test.ts).
• typecheck, test:unit:all (incl. RSC suites — test:unit:rsc runs both flight files), lint, format.

Notes / deferred (still in #582)

• Item 8 (renderRoute synthetic-id stamping) — already addressed by feat(testing): #542 capability + coverage polish for @rangojs/router/testing #581 via a stable WeakMap + a documented input-mutation contract (render-route.tsx), not re-opened here (cloning a handle risks breaking method/identity).
• Item 9 (dispatch executionContext=undefined) — correct for an in-process harness with no platform execution context.
• Cross-seam (prerender tag-union) and self-review (loaderParentId regex) items remain tracked in Gap-review follow-ups (remaining): @rangojs/router/testing refinements + cross-seam + self-review #582.