Turbo Modules: Documentation, migration guide, and public API

Land the user-facing story once the implementation stabilizes.

**Tasks**

* Public API: confirm the option name (`enableTurboModuleTracking`), the integration name (`turboModulesIntegration`), and the options bag (`slowCallThresholdMs`, `ignoreTurboModules`, top-N cap). Run through `yarn api-report` and lock the surface.
* `CHANGELOG.md` under Unreleased / Features.
* Coordinate a docs PR with the Sentry docs repo covering:
* How the feature is enabled (new arch vs. old arch).
* What data lands on spans (attributes, measurements) and on events (tags, contexts).
* How crash reports attribute the active module + method.
* Privacy / overhead notes — especially that method names are sent (so users on strict PII setups know what they're shipping).
* Add a section to `packages/core/AGENTS.md` describing the TurboModule perf logger plumbing so future contributors know where to look.
* Add information about new attributes to https://github.com/getsentry/sentry-conventions (https://getsentry.github.io/sentry-conventions/attributes/)
* Decide and document the deprecation story (if any) for the legacy NativeModules path — most users will migrate to the new arch over the next few RN versions, and the legacy path can be marked as best-effort.
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