Issue creating config/lock files

# LSP Plugins crash Ardour when opening plugin GUI under Flatpak

## Environment

* **OS:** Pop!_OS
* **Ardour:** 9.7.0
* **Ardour installation:** Flatpak from Flathub
* **Ardour Flatpak ID:** `org.ardour.Ardour`
* **LSP Plugins:** 1.2.33
* **LSP Plugins installation:** Flatpak, `org.freedesktop.LinuxAudio.Plugins.LSP`
* **Freedesktop runtime:** 25.08
* **LSP architecture:** x86_64

## Problem

LSP plugins cause Ardour to crash when attempting to open their plugin GUI.

I originally encountered the problem with **LSP Multi-Sampler x48 Stereo**. When creating a MIDI track and selecting the instrument, clicking **Add and Close** causes Ardour to crash.

I then tested an LSP equalizer. The equalizer can be successfully added to an audio track, but clicking the plugin in Ardour's mixer to open its GUI causes Ardour to crash in the same way.

This appears to affect the LSP plugin GUI generally, rather than being specific to Multi-Sampler.

I'm pretty new to trying to use all of this in flatpaks so maybe I have something mismatched?

## Ardour output

When running Ardour from a terminal with:

```bash
flatpak run org.ardour.Ardour
```

I see:

```
Loading ui configuration file /app/etc/ardour9/clearlooks.rc
Ardour: [INFO]: Detecting Audio/MIDI Devices
Found nothing along /home/jesse/.var/app/org.ardour.Ardour/config/ardour9/templates:/app/share/ardour9/templates
Set cursor set to default
fluidsynth: warning: No preset found on channel 9 [bank=128 prog=0]
[WRN] Failed to obtain lock on file '/home/jesse/.var/app/org.ardour.Ardour/config/lsp-plugins/lsp-plugins.lock': code=6
[WRN] Failed to obtain plugin configuration: error=6
```

Initially, the following directory did not exist:

```
~/.var/app/org.ardour.Ardour/config/lsp-plugins/
```

I verified that the Ardour Flatpak configuration directory itself is writable:

```text
drwxr-xr-x 7 jesse jesse ... /home/jesse/.var/app/org.ardour.Ardour
drwxr-xr-x 7 jesse jesse ... /home/jesse/.var/app/org.ardour.Ardour/config
```

I manually created the LSP configuration directory:

```bash
mkdir -p ~/.var/app/org.ardour.Ardour/config/lsp-plugins
```

and verified that files could be created there.

After doing this, LSP successfully created:

```
~/.var/app/org.ardour.Ardour/config/lsp-plugins/lsp-plugins.lock
```

However, Ardour still reports:

```
[WRN] Failed to obtain plugin configuration: error=6
```

The crash still occurs.

## Flatpak plugin installation

The installed relevant Flatpaks are:

```text
Ardour org.ardour.Ardour 9.7.0 stable flathub user
LSP org.freedesktop.LinuxAudio.Plugins.LSP 1.2.33 25.08 flathub user
Stochas VST3/CLAP
org.freedesktop.LinuxAudio.Plugins.Stochas 1.3.13 25.08 flathub user
Surge XT Synthesizer LV2/VST3/CLAP
org.freedesktop.LinuxAudio.Plugins.Surge-XT 1.2.2 22.08 flathub user
Surge XT Synthesizer VST3/CLAP
org.freedesktop.LinuxAudio.Plugins.Surge-XT 1.3.4 25.08 flathub user
```

Inside the Ardour Flatpak sandbox, the LSP extension is present and contains the expected plugin formats:

```text
/app/extensions/Plugins/LSP/lib/clap/lsp-plugins.clap
/app/extensions/Plugins/LSP/lib/vst/lsp-plugins.vst
/app/extensions/Plugins/LSP/lib/vst3/lsp-plugins.vst3
/app/extensions/Plugins/LSP/lv2/lsp-plugins.lv2
```

So this does not appear to be a case where Ardour simply cannot find the LSP plugins.

## Reproduction

1. Install Ardour 9.7.0 from Flathub.
2. Install `org.freedesktop.LinuxAudio.Plugins.LSP` 1.2.33.
3. Launch Ardour with:

```bash
flatpak run org.ardour.Ardour
```
4. Add an LSP plugin to a track.
5. The plugin can be added successfully.
6. Click the plugin in the mixer to open its GUI.
7. Ardour crashes immediately.

The same general behavior occurs with LSP Multi-Sampler x48 Stereo when creating a MIDI track.

## Additional observations

The crash does not appear to be limited to the Multi-Sampler. An LSP equalizer can be added to a track successfully, but opening its GUI causes Ardour to crash.

The LSP configuration directory was initially absent, but manually creating it allowed LSP to create its lock file. This did not resolve the crash.

I have not yet determined whether this is related to the LSP GUI backend, Flatpak sandboxing, Wayland/X11, or a specific plugin format.

A crash backtrace can be provided if useful.

## Question

Is there a known compatibility issue between LSP Plugins 1.2.33, Ardour 9.7.0, and the Freedesktop 25.08 Flatpak runtime that could cause Ardour to crash when an LSP plugin GUI is opened?

I would be happy to provide a GDB backtrace or any additional diagnostic information needed.

Start by reproducing the crash with `flatpak run org.ardour.Ardour`, an LSP plugin, and the GUI-opening steps described. Inspect the sandbox paths under `/app/extensions/Plugins/LSP/` and `~/.var/app/org.ardour.Ardour/config/lsp-plugins/`, then collect the offered GDB backtrace; done means identifying the compatibility or sandbox condition causing the crash and documenting a verified resolution.

Written by the indexing model from the issue text.