Add StorageManager on navigator.storage

Proposal

Implement a minimal StorageManager surface to expose storage usage and persistence semantics for headless and test scenarios.

Scope:

• Add navigator.storage getter.
• Implement estimate returning quota and usage.
• Implement persist and persisted with platform-injected behavior.
• Optional extension point for getDirectory compatibility.
  Technical Notes:
• Use the same service registration pattern as Clipboard and Geolocation.
• Keep persistence decisions delegated to an injected platform interface.
  Acceptance Criteria:
• navigator.storage is available when registered.
• estimate returns deterministic values in tests.
• persist and persisted map to the injected platform implementation.
• Full unit test coverage for success and edge cases.
  Out of Scope:
• Real browser eviction heuristics.
• Full OPFS implementation in first pass.

Description

Add StorageManager on navigator.storage with a minimal but useful subset: estimate, persist, and persisted. The implementation should be platform-injected to support deterministic behavior in tests and host-specific behavior in production.

Background

This complements existing storage APIs by exposing quota/usage and persistence intent, which are common needs in headless automation and integration test environments. It also creates a bridge toward future OPFS support.

Specification

• WHATWG Storage Standard (StorageManager / navigator.storage): https://storage.spec.whatwg.org/
• Relevant methods: estimate(), persist(), persisted()

Start by inspecting the service registration pattern used by Clipboard and Geolocation, then trace how navigator APIs receive platform-injected behavior. Implement the minimal StorageManager surface described in the issue and add unit tests covering deterministic estimate values, persistence success, and edge cases.