fix(dev): the smoke target died silently on a genuinely dead app

Two harness bugs and the finding that exposed them.

**`pidof` exits 1 when it finds nothing**, and under `set -e` a failing
command substitution killed the script before it could print anything
-- rc=1, no output. That was invisible for as long as the app
crash-*looped*, because there is always some pid in that state. It
appeared the moment the app died for good and ActivityManager stopped
respawning it, which is precisely the run you most want output from.

**And an install failure said nothing useful.** Both ways it fails are
about identity rather than the build: INSTALL_FAILED_VERSION_DOWNGRADE
when a bare `make android` (versionCode 1) meets something a versioned
build left behind, and a signature mismatch when a debug-signed local
build meets a release-signed one. Both were hit in one session, and
both are fixed by uninstalling. The target says so now instead of
leaving someone to read the constant name.

The finding: with the startup bug fixed the app reaches the database
and takes SIGSYS on the x86_64 emulator, because modernc.org/libc's
Xlstat64 issues a raw lstat syscall on linux/amd64 and Android's
seccomp filter forbids it -- bionic never issues it. arm64 has no lstat
syscall at all, so ccgo_linux_arm64.go routes Xlstat through fstatat
and is structurally unaffected; Go's own syscall package already used
fstatat on both.

So the default emulator cannot verify this app, and the skill says so
rather than letting the next session read a tombstone as a regression.

.planning/NOTES.md

@@ -2587,3 +2587,59 @@ under the `Wails` tag and are inspectable from `chrome://inspect`;
 production builds compile that out — so a debug APK is the more
 informative one when something is wrong. And the docs recommend
 `build-tools;35.0.0`; 34.0.0 is what is installed here and builds fine.
+
+## The app starts on Android; x86_64 Android cannot run it (2026-08-16)
+
+Two findings, and the second is the one with consequences.
+
+**The startup bug is fixed.** `backend/system`'s `buildUserDirPath`
+switched on `runtime.GOOS` and Android took the `default:` branch, so
+`main()` called `os.Exit(1)` six milliseconds after the JNI bridge came
+up. `main()` now calls
+`system.UseHomeOverride(application.Mobile.StoragePath())` before
+anything asks for a path. `StoragePath()` is `getFilesDir()` on
+Android, Application Support on iOS and `""` on desktop — where
+`UseHomeOverride` is a no-op — so the change needs no build tag and
+alters nothing off mobile. `backend/system` gained no import of the
+Wails application package, deliberately: that is the same constraint
+the `indexbuild` split protects in `backend/events`.
+
+**And then it takes SIGSYS on the x86_64 emulator.**
+
+```
+F/libc: Fatal signal 31 (SIGSYS), code 1 (SYS_SECCOMP), syscall 6
+F/DEBUG: Cause: seccomp prevented call to disallowed x86_64 system call 6
+```
+
+Syscall 6 on x86_64 is `lstat`, and the caller is **not our code and
+not Go's**. Go's `syscall` package already routes both `Stat` and
+`Lstat` through `fstatat` on amd64 *and* arm64. The caller is
+`modernc.org/libc`, which `modernc.org/sqlite` sits on and therefore
+the entire database layer: `libc_linux_amd64.go`'s `Xlstat64` issues
+`unix.Syscall(unix.SYS_LSTAT, …)` directly. Android's seccomp filter
+forbids it because bionic never issues it.
+
+**arm64 is unaffected, structurally rather than by luck.** arm64 has no
+`lstat` syscall at all, so `ccgo_linux_arm64.go`'s `Xlstat` is
+`Xfstatat(…, AT_SYMLINK_NOFOLLOW)` → `SYS_newfstatat` (79), which is
+permitted. `grep -c SYS_LSTAT ccgo_linux_arm64.go` returns 0 against 1
+for amd64.
+
+Three consequences:
+
+- **The default emulator cannot verify this app.** `make android-smoke`
+  on an x86_64 AVD reports a tombstone that says nothing about your
+  change. Verification needs an `arm64-v8a` image (full software
+  emulation on an x86_64 host, so slow) or a real device.
+- **The x86_64 half of the fat APK is dead weight on every Android**,
+  not just emulators — an x86 Chromebook would hit exactly this. It is
+  31 MB of a 27 MB compressed artifact. Dropping it is a real option;
+  keeping it costs size and buys an emulator target that does not work.
+  Not decided here.
+- The failure is at least *legible*. Unlike the `os.Exit` it replaced,
+  SIGSYS leaves a tombstone with a backtrace into `libwails.so`, which
+  is how it was identified in one pass.
+
+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.