docs(migration): document known limits + transform-allowlist from real runs

[danfry1]

What

Expands the Migrating from Jest guide with the empirically-derived reality of a real migration, from friction audits across the obytes, react-native-paper, and rneui suites:

• New "Known limits" section — assertions coupled to Jest's RN mock internals that can't reproduce under a real-RN engine, each with a recommended action:
  • jest.spyOn(View.prototype, 'measure'/'measureInWindow') (real RN's View is a forwardRef, no prototype method)
  • jest.mock('react-native/Libraries/…') of internal submodules + spying them (externalized RN isn't intercepted)
  • raw source-shape assertions (real RN normalizes source)
  • jest.mock('react-native', …) nested in beforeAll/describe callbacks (Jest doesn't hoist those; vitest-native applies mocks more consistently)
• Expo-core caveat — suites importing Expo core pull in dev-server/init plumbing that expects a Metro connection and may not collect; suites using Expo modules via the expo preset are unaffected.
• Concrete transform allowlist guidance — the JSX-in-.js third-party-lib parse error (a lib pointing its react-native/module field at untranspiled src/) and the fix (add the package to transform), as the direct replacement for transformIgnorePatterns; also noting vi.mock needs the lib in the Vite graph.

Applied to both the website guide and the packaged docs/ copy so they stay consistent.

Why

The guide already covered the mechanical steps but set the expectation that remaining failures are "usually a missing query update." The audits showed a distinct, nameable class of Jest-mock-coupled assertions that won't port — documenting them sets honest expectations and tells migrators what to rewrite vs. chase.

Validation

• vitepress build website renders cleanly with these changes.
• Docs-only; no code paths touched.

Note (separate issue, not in this PR)

While validating, found that website/guide/fidelity.md (a generated file from the fidelity-report pipeline) contains a bare <Text> in a table cell that VitePress parses as an unclosed tag, which breaks vitepress build on main (and thus the Pages deploy). The fix belongs in the report generator (escape/backtick-wrap angle-bracket tokens); filing separately.