refactor(simu): make settingsPath a true overlay FS over sdPath

[raphaelcoeffic]

Replace the prefix-based path routing (/RADIO, /MODELS → settingsPath, everything else → sdPath) with a true overlay filesystem where settingsPath is layered on top of a read-only sdPath:

• Reads: resolve in settingsPath first, fall back to sdPath
• Writes: always go to settingsPath (w/ copy-on-write for append mode to file existing in sdPath)
• Directory listings: merge entries from both layers, settingsPath wins on case-insensitive duplicates
• Parent directories are auto-created in settingsPath only when they exist in at least one layer, preserving accurate FAT FS semantics

This enables Companion to provide files (e.g. model images in /IMAGES) in settingsPath while still seeing files from sdPath in the same directories (see #7234).

[elecpower]

Testing performed using radio profile TX16SMK3 with sd path configured.
Loaded an etx file containing the model and the linked image.
Ensured the sd path/IMAGES did not contain the linked model image.
Simulated the radio.
Checked terminal output to confirm model image written to the temporary settings directory.
In the sim UI setup.
Screen 1.
Setup widgets.
Added Model Info widget.
Returned from widget settings and model image displayed.
Returned to the main screen.
Model setup
Model settings
Opened model image dropdown and files from settings patch and sd path displayed. Selected a file from the sd path and returned to the main screen and the new image displayed.
Repeated previous step and selected the image on the settings path and that image displayed on the main screen.

Repeated the tests with a file with the same name as the model image but containing different image and saved to sd path/IMAGES.
The dropdown list did no contain duplicates.
The settings path image was displayed when selected from the dropdown.

[elecpower]

Todo test stand alone simulator

[elecpower]

Tested stand alone simulator
File mode where etx file had copy of image. The radio profile sd path had the same file name but a different image. Displayed etx version of the image. Pass
Folder mode using the radio profile sd path. The sd path image displayed. Pass
SD Image Path mode same as Folder mode. Pass

[elecpower]

Getting some file save errors on closing stand alone simulator. Trying to determine if my environment or code as it was working.

[raphaelcoeffic]

Getting some file save errors on closing stand alone simulator. Trying to determine if my environment or code as it was working.

What kind of errors exactly? It cannot save the model/radio files?

[elecpower]

What kind of errors exactly? It cannot save the model/radio files?

Saving the images is failing and it appears it always was but until I improved the checks and debug messages it was silently failing and continuing. Well better to find that now.

[elecpower]

Found the problem in the save image PR.