feat: add privacy string accessors

[ethanpschoen]

Description of this change

Adds accessors for the IAB privacy strings the tag writes to native storage: TCF TC string, US Privacy string, GPP header string, and a generic getSavedString helper.

The util barrel export (util/index.ts) also picks up ./deviceLocale from the PR below this one in the stack, since both additions land in the same import block.

Part 2/5 of the split of #126. Full stack: #137 → #138 → #139 → #140 → #141.

Review finding fixed here: Getters miss preferenceStorage values — getSavedString and the IAB helpers always read via nativeStorage, so they never saw values persisted through a custom preferenceStorage backend (required for Expo Android per the README). Fixed by accepting an optional reader parameter, defaulting to the existing nativeStorage.read. The provider wiring that builds a reader from the configured preferenceStorage and binds these accessors to it lands in #141.

Why is this change being made?

• Chore (non-functional changes)
• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How was this tested? How can the reviewer verify your testing?

npm run lint and npm run typecheck pass standalone on this branch. No dedicated unit tests exist for privacyStrings.ts on the source branch — the functions are thin wrappers around nativeStorage, exercised indirectly once the KetchService members are exposed in #141.

The full 5-PR stack was verified as a faithful, complete split of #126 before publishing: git diff --exit-code <top-of-stack> origin/ethanpschoen/feat/headless-api is empty (exit 0), and the top of the stack passes the full local gate (lint, typecheck, build, npm test).

Related issues

Refs #126.

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have evaluated the security impact of this change, and OWASP Secure Coding Practices have been observed.
• I have informed stakeholders of my changes.

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit 98a034a. Configure here.}