Stop Gradle installing on every checkout's emulator at once

The adb wrapper can only aim a call that goes through it, and Gradle's
Android tasks do not: installDebug, uninstallDebug and connectedAndroidTest
ask the adb server for every attached device and act on all of them. One
session ran installDebug with two emulators up and replaced the app on both,
which Gradle reported as success and the other session read as its own build
never landing.

`emu check` answers the question the wrapper already answers for adb --
would a command that reaches every attached device reach a stranger's
emulator from here -- and install.sh links a Gradle init script into
~/.gradle/init.d so every build on this machine asks it, including checkouts
nobody has adopted it in.

It refuses narrowly: a lone emulator, a physical phone and a caller who
named a device are all fine. ANDROID_SERIAL naming another checkout's
emulator is not, because that is what a serial exported into a long-lived
shell decays into once an emulator restarts and another's takes the port.

README.md

@@ -14,11 +14,12 @@ anything it replaces as `<name>.bak`.
 
 | | |
 |---|---|
-| `emu` | this checkout's emulator: `name`, `serial`, `list`, `up`, `down` |
+| `emu` | this checkout's emulator: `name`, `serial`, `list`, `up`, `down`, `check` |
 | `adb` | adb, aimed at this checkout's emulator, with screenshots scaled |
 | `ui-trace` | records a screen as text at 60Hz, and drives it |
 | `lib/project-avd.sh` | the rules all three share |
 | `share/ui-trace/` | the device recorder (`UiTrace.java`) and its build |
+| `share/gradle-init/` | the init script that stops Gradle installing on every device |
 
 ## One emulator per checkout, named after it
 
@@ -40,6 +41,38 @@ about one device (`adb devices`) all turn the defaulting off, and nothing is
 guessed when this checkout's emulator is not running: adb's own error is
 better than a wrapper picking a stranger's device.
 
+## Gradle is the hole the wrapper cannot cover
+
+`adb` can only aim a call that goes through it, and Gradle's Android tasks
+do not: `installDebug`, `uninstallDebug` and `connectedAndroidTest` ask the
+adb server for every attached device and act on all of them. On 2026-08-31
+one session ran `./gradlew :androidApp:installDebug` with two emulators up
+and replaced the app on both. Gradle reported success; from the other
+session it looked like its own build had never landed.
+
+So `install.sh` also links `share/gradle-init/emulator-tools.gradle` into
+`~/.gradle/init.d`, where Gradle applies it to **every** build on this
+machine — including checkouts nobody has adopted it in, which are exactly
+the ones this is protecting. Before any of those tasks runs it calls
+
+    emu check
+
+in the project's own directory. That is the same rule the `adb` wrapper
+follows, in one place: what must not be touched is *another checkout's
+emulator*. A physical phone is nobody's checkout, one emulator on its own is
+nobody else's business, and a caller who named a device has already narrowed
+the fan-out to it — so `check` passes all of those, and refuses only when a
+stranger's emulator is in range. `ANDROID_SERIAL` pointing at one is refused
+too, because that is what a serial exported into a long-lived shell decays
+into once an emulator restarts and another checkout's takes the port.
+
+Say which device you mean at the moment you use it rather than exporting it:
+
+    ANDROID_SERIAL=$(emu serial) ./gradlew :androidApp:installDebug
+
+`EMU_ANY_DEVICE=1` means every attached device anyway, for the case that is
+genuinely what you want.
+
 ## Rendering on the GPU with no display
 
 This machine has no screen, and for a long time that meant the emulator