docs(android): an arm64 image will not run on an x86_64 host

Emulator 37 refuses cross-architecture emulation outright -- "Avd's CPU
Architecture 'arm64' is not supported by the QEMU2 emulator on x86_64
host" -- and there is no flag for it. Google dropped it.

That matters because the previous commit's finding points at arm64 as
the ABI that works, so the obvious next move is to boot an arm64 AVD,
and the obvious next move costs a 3.8 GB download before it fails.
Written down so the next session does not spend it.

The consequence is stated rather than hidden: the claim that arm64
avoids the seccomp trap rests on reading modernc's two code paths, not
on having run it. Verifying it needs an arm64 host, a physical device
or adb connect.

.planning/NOTES.md

@@ -2643,3 +2643,19 @@ Three consequences:
 Worth knowing for anything else that reaches for a pure-Go C library:
 this class of bug is invisible to every build and every desktop test,
 and appears only under a platform's syscall filter.
+
+### …and the arm64 emulator is not an option on an x86_64 host
+
+Emulator 37.1.11 refuses outright, after the 3.8 GB image download:
+
+```
+FATAL | Avd's CPU Architecture 'arm64' is not supported by the QEMU2
+        emulator on x86_64 host. System image must match the host
+        architecture.
+```
+
+Google dropped cross-architecture emulation and there is no flag for
+it. So the arm64 claim above rests on reading modernc's two code paths,
+not on having run it: verifying the shipped ABI needs an arm64 host, a
+physical device, or `adb connect` to one. The image was deleted again;
+do not re-download it.