Follow-up to #533: capability + coverage polish for @rangojs/router/testing #542

Follow-up to #533 (feat(testing): consumer testing primitives). The module's abstractions and core capabilities are validated (4-lens usefulness review: medium-high, "ship it"; the high-value core runs real router internals). #533 ships the message + limits (test-pyramid framing + a complete "deliberately don't cover" trap table). This issue tracks the capability + coverage polish — turning documented false-confidence traps into loud failures, and closing the biggest capability gap — so a misleading test errors instead of passing green.

1. Make silent false-confidence traps fail loud (not just documented)

These currently pass green while proving nothing:

• renderRoute navigation frozen at idle — navigate() bypasses the lifecycle, so useNavigation/useLinkStatus/useAction non-idle states are invisible. Warn/throw when a test relies on a non-idle state.
• "use cache" run without a seeded cacheStore — registerCachedFunction bypasses, silently exercising the uncached path. Emit a warning that the cached path is not being tested.
• Flight getRequestContext()/cookies() alias trap — without the rsc-project rangoTestAliases, a context-reading handler hits the throwing stub and surfaces as opaque tree: undefined. Detect this and emit an actionable "add rangoTestAliases to your rsc project" error.
• renderRoute wrong mount prefix — runtime-silent; validate the prefix against the routes.

2. Close the biggest capability gap

• Full-app router import in bare tests — a router using Prerender()/createLoader() throws "missing $$id" (plugin-injected at build). Either support injected ids in bare tests, OR promote the Prerender-free-include recipe to the canonical quickstart dispatch/assertGeneratedRoutesMatch pattern (it is documented but mid-doc).

3. Trim / de-emphasize marginal surface

• Steer users to renderServerTree (inspect props/boundaries) over raw renderToFlightString wire-string matchers for assertions.
• Consolidate the dual cache-telemetry paths (parseCacheHeader/assertCacheStatus vs createCacheSink/filterCacheDecisions).

Already done in #533

• Lead docs with the test-pyramid framing.
• Complete the "deliberately don't cover" trap table — added rows for: "use cache" uncached-path, whole-app router import block (Prerender/$$id), and the ctx.rendered() barrier-wiring loader trap.

Relates to #533.