docs(mobile/4): document env-gated plugin feature bundles

[shanerbaner82]

What

Documents the features key in nativephp.json, which lets a plugin declare optional pieces of itself behind an environment variable so apps compile in only what they enable.

New page: Plugins → Feature Bundles (order 725, between Permissions & Dependencies and Advanced Configuration), plus a short cross-reference from permissions-dependencies.md so someone declaring a heavy SDK finds it.

Why

A plugin wrapping a large SDK forces every app to pay for every capability. nativephp/mobile-firebase is the motivating case: push, Crashlytics, App Check, Performance, and In-App Messaging all in one plugin — an app that only wants push shouldn't link the crash reporter and attestation provider.

What the page covers

• The problem, then the manifest shape (using the real Crashlytics bundle as the example)
• A table of what a bundle may declare, and the merge semantics — lists concatenate, maps merge key-wise, dependencies recurses
• Why feature sources live in resources/features/<feature>/<platform>/ rather than a subdirectory of resources/ios: those roots are copied recursively, so a gated-off subdirectory would still compile in and reference SDK products that were never linked. This is the constraint a plugin author is most likely to get wrong first.
• Sharing one SDK package across features — declare the same url with only the products you need; the compiler attaches them to the existing package reference
• Truthiness rules, and an explicit "what a disabled feature costs: nothing" list
• A callout on the version requirement: on an older core the key is silently ignored, so pin the constraint and add a PluginManifest test that fails loudly
• A nudge to document each variable in the plugin README, since app developers can't discover features by reading code

⚠️ Do not merge before the core PR

This documents behavior from NativePHP/mobile-air#289, which is still open. Merging ahead of it would publish docs for a feature nobody has.

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