Add user identity (setUserId / logout)

[ahsanalidotme]

Closes linktrail-io/linktrail-backend#112

Backend PR (deploy first, SDKs send userId which older backends reject with 400): https://github.com/linktrail-io/linktrail-backend/pull/139
The backend PR (POST /v1/events/identify + optional userId on events) must be deployed before this is released.

What

LinkTrail.setUserId(id): Promise<void>, LinkTrail.logout(): Promise<void>, LinkTrail.getUserId(): Promise<string | null>.

• Validation ^[A-Za-z0-9_.:-]{1,128}$; invalid id rejects with code: 'invalid_user_id', nothing stored/sent.
• Persisted across launches (wrapper-owned: UserDefaults / SharedPreferences, no new dependency).
• Consent: the id is never forwarded while consent is denied/undecided; forwarded once on grant (or at configure when requireConsent: false); revoking clears it on the native side. logout() clears locally, no network call.
• Logic lives in new src/identity.ts (pure TS, injected bridge). Tests: npm test (node:test + tsx; 11 tests: validation, persistence, consent gating, logout, failure swallowing).

Design decision and limits

The native SDKs perform all HTTP and own the device id (internal), so the JS layer cannot call /v1/events/identify or add userId to events itself. I did not depend on unreleased native APIs or bump pins. Instead the wrapper owns identity state and hands the id to the native SDK through a new forwardUserId bridge method:

• Android: reflective lookup of setUserId(String) / logout() on the SDK, so it compiles against io.linktrail:sdk:0.0.5 and delivers automatically once the pin moves to a release that has them (method names assumed from the issue; must match the other agent's Android PR).
• iOS: Swift cannot probe for a missing method, so forwardUserId returns false for now. Needs a one-line wrapper change when the pod pin moves to a release with identity.
• With the currently pinned native SDKs nothing is transmitted: the id is validated, persisted and consent-gated only. Requires native SDK >= the first release with identity support (version TBD).

Checks

npm run typecheck, npm run lint (0 errors, warnings pre-existing/no-deep-imports), npm run prepare, codegen validation, npm test all pass locally.
CI does not run tests; adding npm test to .github/workflows/ci.yml is a follow-up (workflow files are not touched here).

Unverified

• No device/simulator run; Kotlin/Swift/ObjC++ changes were not compiled (only codegen validated).
• Reflection method names vs. the native SDK PRs; end-to-end delivery to the backend.
• package-lock.json changed only for the new tsx devDependency.

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