build(android): drop the x86_64 ABI, which no Android can run

The fat APK's second half was 31 MB that cannot execute on any Android
device. modernc.org/libc's Xlstat64 issues a raw lstat syscall on
linux/amd64, and Android's seccomp policy forbids it because bionic
never issues it, so the process takes SIGSYS the first time anything
touches the database -- which for this app is startup. That is every
x86_64 Android, x86 Chromebooks included, not merely the emulator.
arm64 is structurally unaffected: the architecture has no lstat syscall
at all, so modernc routes through fstatat.

27,059,130 bytes to 15,898,465, and one lib/ entry.

Three places had to agree, and the third is what would have made this a
silent no-op: abiFilters (what Gradle packages), android:package rather
than package:fat (what Go *compiles* -- otherwise the library is still
built and then discarded), and the native-code assertion in CI. That
assertion is anchored, `native-code: 'arm64-v8a'$`, because without the
anchor it also matches the fat APK's line and would pass on exactly the
thing it exists to catch. Checked against a real artifact.

Adding the ABI back, if modernc ever fixes Xlstat64, is those same
three edits.

Makefile

@@ -52,8 +52,13 @@ ANDROID_SDK ?= $(HOME)/Android/Sdk
 ANDROID_NDK ?= /opt/android-ndk
 ANDROID_ENV := ANDROID_HOME=$(ANDROID_SDK) ANDROID_SDK_ROOT=$(ANDROID_SDK) ANDROID_NDK_HOME=$(ANDROID_NDK)
 
-android: build-frontend ## Build the fat APK (arm64 + x86_64) into bin/
-	@$(ANDROID_ENV) PATH="$(TOOLBIN):$$PATH" go tool wails3 task android:package:fat
+# `package`, not `package:fat`: x86_64 Android cannot run this app at
+# all (modernc's raw lstat vs Android's seccomp -- see
+# android-tier.md), so the second ABI was ~31 MB that could not run
+# anywhere.  app/build.gradle's abiFilters says the same thing to
+# Gradle; both have to agree or the .so is built and then dropped.
+android: build-frontend ## Build the arm64 APK into bin/
+	@$(ANDROID_ENV) PATH="$(TOOLBIN):$$PATH" go tool wails3 task android:package
 
 android-setup: ## Install the SDK pieces and create the AVD (once, ~3.5GB)
 	@$(ANDROID_ENV) ./scripts/android-emulator.sh setup