feat(testing): loud-fail the alias trap + bare-import Prerender/createLoader/Static (#542) - #543

…tes [skip ci]

The Setup-section import-contract blockquote was reflowed badly (lost the > quote
prefix on continuation lines and ate spaces around inline code: $$idis,
realrouter.tsx, pluginvirtual:). Convert it to two normal paragraphs so it renders
correctly. Also clear stale Prerender-free / blocked-by-Prerender stragglers in
cloudflare-basic + vite-rsc-demo FINDINGS to the new contract (the whole-router
FILE import is blocked by app page-module deps / plugin virtual: modules, not by
handler $$id, which now falls back).

… production throw)

Review finding (Medium): gating the missing-$$id throw on NODE_ENV==="development"
meant the runtime fallback fired in ANY non-dev runtime, including a production
`vite build`. For an UNSUPPORTED handler shape (e.g. `export let X = Static(...)`)
the Vite plugin leaves the id un-injected and untracked with no warning, so a
production build that previously threw loud ("missing $$id") would silently assign
__rango_runtime_static_0 and degrade to a runtime static/prerender miss. The
"fallback never fires under the plugin" assumption only held for SUPPORTED shapes.

Fix: gate the fallback on `process.env.VITEST` — the only signal true in both
vitest projects (the RSC project forces NODE_ENV=production) yet absent in a real
build. The fallback now fires ONLY under a test runner; a missing id in dev OR a
production build throws, restoring the pre-PR safety net. Applied to all five
create-functions (handle.ts, loader.ts, loader.rsc.ts, prerender.ts,
static-handler.ts) — they shared the identical gate and the same latent bug.

Tests: the id-guard / fallback tests now toggle VITEST (not NODE_ENV) to exercise
the throw ("throws outside a test runner" cases). Docs (testing/vitest.ts JSDoc +
docs/testing.md) note the throw-on-unsupported-shape in a real build. Gate:
typecheck 12/12, test:unit:all EXIT=0 (router 3495 + 3 todo, cloudflare-basic 39,
RSC 27), lint 0/0, format.

Relates to #533, #542.

…gnostic dev/test-only

Review finding (Medium): the previous `!process.env.VITEST` guard shipped to
production with two problems. (1) It is a BARE `process.env` access — a runtime
without `process` (browser, certain edge/worker RSC envs) can ReferenceError before
reaching the intended Rango error. (2) Unlike `process.env.NODE_ENV` (folded by the
app build `define`), `VITEST` is not folded, so switching to it un-DCE'd the rich
stack-parsing `missingInjectedIdError` diagnostic — it shipped in every prod chunk.

Fix:
- New runtime-safe helper `isUnderTestRunner()` = `!!globalThis.process?.env?.VITEST`
  (optional-chained — never a bare `process.env`). Used by all five create-fns.
- Restructure handle.ts / loader.ts / loader.rsc.ts so the rich diagnostic stays
  behind `process.env.NODE_ENV !== "production"` (build-folded -> DCE'd, tree-shakes
  missing-id-error.ts out) with a SMALL production throw in its place. Static() /
  Prerender() already throw small messages — only the runtime-safe check changed.

Verified via `pnpm --filter rangojs-e2e-mini build` + chunk inspection: shipped .js
uses `globalThis.process?.env?.VITEST` / `{}.VITEST` (no bare access); the rich-only
markers ("Unsupported create", "created at", stack parsing, node_modules filtering)
are DCE'd (0 across all chunks); only the small throw ships.

Tests toggle VITEST (not NODE_ENV); added a prod-small-throw test (NODE_ENV=production
+ no VITEST -> small, non-stack-parsing message). Gate: typecheck 12/12, test:unit:all
EXIT=0 (router 3496 + 3 todo, RSC 27), lint 0/0, format.

Relates to #533, #542.