docs+ci: sync trust surfaces with the actually-gated RN range and engine reality

[danfry1]

Problem

Three drift classes on the project's trust surfaces:

1. The published fidelity page understated CI coverage. The lockfile has shipped React Native 0.86 since fix(compat): mock react-native 0.86 top-level exports #38, so the full gate runs it on every push — but native-rn-matrix.yml (which fidelity-report.mjs derives the published range from, by design) still stopped at 0.85. The fidelity page, both READMEs, and five website pages all claimed 0.81–0.85.
2. The fidelity page never said which RN version produced the committed probe numbers. The crosscheck report captures reactNativeVersion; the page template dropped it.
3. Website pages taught code that throws under the default engine. The quick start's "Control device state" example used setPlatform('android'), which throws under engine: 'native' (the platform is fixed when the real module graph loads). Three more pages carried the same unqualified claim, setInsets() was undocumented on the website, and the mocks option page lacked its mock-engine-only caveat (the plugin throws a configuration error when it's set under native).

Change

• Add the RN 0.86 row to the matrix, with a header note documenting the matrix→fidelity-page coupling so the newest row stays in sync with the lockfile.
• Render the generating RN version into the fidelity summary; regenerate the page and badge (fidelity:check passes).
• Update the supported-range claim to 0.81–0.86 across both READMEs and the website.
• Quick start now demonstrates setDimensions/setColorScheme (engine-agnostic) and explains the config/projects route for platform testing; helpers.md gets an engine-caveat warning on setPlatform and documents setInsets; plugin-options.md corrects the platform guidance and adds the mocks engine caveat; the feature list on the guide index reflects which helpers work under both engines.

All behavioral claims verified against the code (native/setup.mjs control surface, plugin.ts mocks rejection, helpers.ts setInsets).