Zypak spawn strategy broken: Electron apps spawn processes but no window appears

## Summary

All Electron flatpak apps using zypak's spawn strategy launch processes but never produce a visible window. Non-Electron flatpaks on the same system work fine. Bypassing zypak with `--command= --no-sandbox` works.

## Environment

- Flatpak 1.16.3
- bubblewrap 0.11.0
- Kernel 6.19.9 (Arch-based)
- Wayland (KDE Plasma 6)
- org.freedesktop.Platform 25.08

## Affected apps

Tested with Brave, Mattermost Desktop, and Bitwarden — all exhibit the same behavior. A GTK flatpak on GNOME runtime works fine, confirming the issue is in the zypak/Electron layer.

## Observed behavior

```
$ flatpak run com.brave.Browser
error: linkat: No such file or directory
[2 preload-host-spawn-strategy] Failed to let stub process know of exit: Bad file descriptor (errno 9)
```

Processes are created (verified via `ps`) but no window appears. With `ZYPAK_DEBUG=1`, the spawn strategy completes D-Bus portal calls and spawns a child PID, but the zygote child immediately reports "Got supervisor exit message" and exits.

Both the older zypak (~540KB helper, e.g. Mattermost) and the newer rebuild (~4MB helper, e.g. Brave) are affected.

## Workaround

Bypassing zypak entirely works:

```
flatpak run --command= --no-sandbox
```

Flatpak's own sandbox still applies.

## Notes

- Also filed at https://github.com/refi64/zypak/issues/46 but that repo appears inactive
- The `linkat` error is from flatpak's libglnx (non-fatal) and appears to be unrelated to the spawn failure