feat(tools): add screen-fingerprint, a stable answer to "which screen is this"

[hubgan]

Stacked on #726.

Why an element is a weak proxy for a screen

Every screen check argent had went through one element of the UI tree, and that is unreliable in four separate ways:

• a shared header matches on two different screens;
• a positional id encodes how many siblings exist, not what the thing is;
• during a push or a modal presentation, source and destination are in the tree together, so a destination landmark matches while the destination is still animating in;
• the vocabulary of the tree is source-dependent — iOS describe prefers the ax-service and falls back to native-devtools when the AX read comes back empty, flipping labels to testIDs mid-session.

What this adds

screen-fingerprint reads the app's focused React Navigation route path and returns it as "HomeTab>Profile".

Because it comes from the app's own navigation state it is independent of the tree source, of content (/user/alice and /user/bob share a route), and of locale. One route names one screen.

It is read the same black-box way argent already reads the RN component tree: walk the fiber tree via __REACT_DEVTOOLS_GLOBAL_HOOK__ and collect the focused route/navigation prop pairs. No app cooperation is required — the app does not have to export its navigationRef. The probe is self-contained, defensive, and fiber-count bounded so it cannot hang navigation.

A read-only, device-only probe: nothing is tapped, no file is touched.

Two limits, stated rather than papered over

Both are load-bearing for callers, so they are documented in the API and in the skill:

1. A native overlay above an RN screen — a system permission alert, a share sheet, an RN <Modal> — does not change the focused route, so the probe reports the screen beneath it. Route identity answers "which screen", never "is anything covering it".
2. Navigation state commits when the navigator commits, which is before the transition finishes animating. It is not a readiness signal either.

Answers that mean different things

• available: false — this build can never have a reader (release build, fully native app, Chromium, Metro down). Recognize the screen by a destination-only element instead.
• route: null with available: true — no focused route at this instant: a native screen, or a transition still in flight. Let it settle and probe again.

app_id is required, and guards against reading a foreign app's Metro on the same port.

metroServerRunning

Added alongside, and deliberately not discoverMetro — which also requires at least one CDP target and throws otherwise. That distinction is the whole point: a Metro serving one app reports an empty target list for several seconds after that app relaunches, so "no targets" is the normal post-launch state, not a down server. Answering the up/down question with target discovery told authors to start a server that was already running.

Skill/docs

argent-device-interact gains the tool and its two limits; the routing rules gain a line pointing at it for "which screen is the app on". The await-screen-idle row joins the same table — it documents a tool that already exists and belongs beside this one.

[hubgan]

Closing: the route fingerprint is not being taken forward.

Reading the focused React Navigation route looked like a stable answer to "which screen is this", but it is weaker than it appears. Two screens presented inside one route report the same fingerprint, so a gate on it passes whether or not the navigation happened - and the vacuous case is invisible at authoring time. It also needs a Metro-served debuggable build, so Chromium, release builds and fully native screens fall back to an element check anyway, which is what every flow then has to be written against.

#728 keeps the half that earned its place, await: { idle: true }, and now proves identity with an element that exists only on the destination. That works on every platform and cannot be vacuously satisfied by a shared route.

The metroServerRunning helper added here went with it; nothing else depended on it.