feat: add frameLocator support for cross-origin iframe interaction

[giulio-leone]

Summary

Add a new framelocator action that uses Playwright's frameLocator() API to enable interaction with cross-origin iframes. The existing frame command uses page.frame() / contentFrame() which doesn't work across origins due to browser security restrictions.

Changes

• src/browser.ts: Add activeFrameLocator property, setFrameLocator() method, and modify getLocator() to scope through frameLocator() when active. Also clear frame locator when switchToMainFrame() is called.
• src/types.ts: Add FrameLocatorCommand interface
• src/protocol.ts: Add framelocator schema to validation and discriminated union
• src/actions.ts: Add framelocator case in dispatch and handleFrameLocator handler
• cli/src/commands.rs: Add frame locator <selector> CLI parsing

Usage

# Set frame locator for a cross-origin iframe
frame locator iframe#payment

# Now all subsequent commands target elements inside the iframe
click button#submit
fill input#card-number 4242424242424242

# Return to main frame
frame main

How it works

Playwright's frameLocator() is a lazy API that doesn't require the frame to be loaded at the time of the call and works with cross-origin iframes (unlike frame()). When a frame locator is active, getLocator() scopes all element lookups through page.frameLocator(selector).locator(...).

Closes #279

[vercel]

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