Make Espresso waits say what they are waiting for

[parawanderer]

Fixes the instrumented job failing on main after #62.

Three CI failures in a row, all the same shape, none of them product bugs.

What was wrong

The waiting helper was copied into three test classes. A flake was found and
fixed in one; the identical code in the others kept failing. That is literally
what happened — the keyboard-over-the-button fix went into one login test, and
CI then failed on its sibling.

The retry was also wrong in a way copying could only spread. Retrying until
nothing throws cannot tell two opposite situations apart:

• the tap missed — the soft keyboard was still over the button — and must be
  retried
• the tap worked, and what it started has already torn the screen down, so
  Espresso throws NoActivityResumedException from the very same call

Retrying the second turns one success into fifty failures. That is what broke
the post-merge run: theConfiguredServerIsWhatTheSignInActuallyUses signs in
immediately, so the click that worked also ended the activity.

The fix

One Eventually class, two methods, and picking the wrong one is the bug:

• Eventually.check(...) — assertions and waiting for views. Espresso waits for
  the main thread and nothing else, while every step here runs on an Rx
  scheduler.
• Eventually.perform(what, tookEffect, action) — anything that changes the
  screen. The caller states what "it worked" means, usually a fake having been
  called, and it stops the moment that holds regardless of what was thrown.

Eventually.perform("the sign in button", () -> apple.timesCalled("login") > 0,
        () -> onView(withId(R.id.login_button_main)).perform(click()));

Documented in AGENTS.md, along with two rules from the same failures: one
ViewAction per perform when the action may finish the flow, and a GONE
view still matches withId.

Verification

Two consecutive full runs on the managed device, 106 tests, no failures. Since
the whole point is that this used to pass locally and fail in CI, the run on
this PR is the one that counts.

🤖 Generated with Claude Code