Locators inside a WebView, and find answering from the page (CHALLENGES 312)

[lana-20]

Locators worked only on native screens: inside a WebView every action took a ref and nothing else, so a `mobium test` file, which has no refs, could not touch a page. This is ROADMAP locators item 4.

• `css=` with a selector, optionally after a frame's path as `map` prints it (`css=iframe#f >> #id`).
• text, label, testid, role and class, matched against the page map by a native locator's rules: a quoted value is the whole text, a whole match is preferred among several, and `,nth=` counts down the page.
• The element found goes through every check a web action already makes. `within` and `path` are refused there as unsupported, naming `css=`.
• CHALLENGES 312: in a WebView, `find` read the native screen and listed the app's Back button. It now matches the page's elements and gives each the ref `map` gives it, which also fixes the inspector's Find.
• A refusal among look-alikes names each one's frame. A selector written without its frame is answered with the path that reaches what it matched.

Checked on MobiumApp, each tap judged by what the page said it reached, on the Android 15 emulator and the iPhone 17 Pro simulator:

• Actionability page: 19 of 19 cases on each device.
• Frames page: taps by path reached the same-origin, nested and cross-origin frames on each device.

`wait` inside a WebView is still refused; ROADMAP has it as the next step. `make ci` passes.