fix(playwright): persist storageState cookies and localStorage

[xrip]

What changed

Adds Playwright BrowserContext.storageState compatibility for cookies and origin-scoped localStorage. Storage is shared by pages in one context and isolated between contexts. Adds Network.clearBrowserCache as a protocol-compatible no-op and Storage.clearCookies, which Playwright uses while loading state. Adds an offline Playwright regression test and documents save/restore usage.

Related to #629. The existing issue tracks persistent session state for obscura mcp; this PR covers the standard Playwright storage-state path.

Validation

• cargo build --release -p obscura-cli --no-default-features
• node crates/obscura-cdp/tests/playwright_storage_state.mjs (pass with Playwright 1.62.1)
• cargo nextest run --release --no-default-features -p obscura-cdp -E 'not test(max_connections_refuses_then_recovers)' (114 passed, 4 skipped)
• The existing max_connections_refuses_then_recovers test was also run alone and failed with a Windows TCP connection reset; it does not touch the changed code.
• node --check crates/obscura-cdp/tests/playwright_storage_state.mjs

Rendering

Not applicable.

Performance

No expected hot-path impact. localStorage has bounded per-origin, total-byte, and origin-count limits.

Checklist

• The change is focused and does not remove existing behavior without justification.
• Tests cover the failure or feature.
• Existing relevant tests pass in the no-render configuration.
• I checked for CPU, latency, and memory regressions.
• Public API and user-facing behavior changes are documented.