docs(SDK-971): backfill phase 3

[mariechatfield]

Summary

Backfills TSDoc coverage for Phase 3 directories per the SDK-971 documentation epic. All three directories can run in parallel and have no dependency on Phases 1–2.

Changes

• src/types/ — @public TSDoc for SDKError, SDKErrorCategory, SDKErrorCategories, SDKFieldError, SDKHooks, ObservabilityHook, ObservabilityError, ObservabilityMetric, ObservabilityMetricUnit, SanitizationConfig; SDKInternalError and normalizeToSDKError marked @internal (confirmed leaked implementation details)
• src/shared/ — @public TSDoc for componentEvents, EventType, all domain event sub-objects, and onboarding status constants; 42 internal constants marked @internal; 6 genuinely unused constants removed by knip
• src/partner-hook-utils/ — @public TSDoc for composeErrorHandler, composeSubmitHandler, SDKFormProvider, HookFieldProps types, and all types.ts exports; internal implementation helpers marked @internal; 11 leaked barrel re-exports removed from form/index.ts by knip

Related

• Jira ticket: SDK-971
• Related PRs: docs(SDK-971): tsdoc models and internal hooks #1996
• Base branch: marie/improve-tsdoc-skills

Testing

npm run build          # clean build, no type errors
npx eslint src/types/ src/shared/ src/partner-hook-utils/  # no errors
npm run api-report:derive  # no new ae-forgotten-export warnings; ae-internal-missing-underscore on SDKInternalError/normalizeToSDKError is expected