Add GPU backend probe with automatic fallback for older GPUs

Older GPUs without usable DirectX 12 Feature Level 12_0 or working
Vulkan support (e.g. legacy Intel iGPUs and other old parts), and old
GLES drivers whose shader compiler rejects iced's own shaders, crash
at startup or on the first drawing viewport: StagingBelt aborts in
Buffer::get_mapped_range (panic=abort, silent exit) on the DX12 path,
and shader compilation fails inside iced's gradient pipeline on the
GLES path. Selecting a working backend via WGPU_BACKEND before iced
boots avoids both classes.

- Probe each candidate backend (Windows: dx12,vulkan,gl; Linux:
  vulkan,gl) in an isolated --gpu-probe child: device request,
  staging-style mapped upload, render pipeline creation (its fragment
  shader reads a private vec4 array, catching old-Mesa compile
  failures), offscreen render and readback. First fully-passing
  backend wins; explicit --backend / WGPU_BACKEND / --safe-mode
  always win, and Windows keeps preferring DX12/Vulkan (#55).
- Software adapters (WARP/llvmpipe/SwiftShader) are the last resort
  before a best-effort GL attempt: slow but alive.
- The crash sentinel names the attempted backend so only implicated
  backends are skipped next launch.
- Winning probes report adapter limits; GPUs without shader storage
  buffers get the packed compatibility renderer automatically, with
  a startup command-line notice so the switch is never silent.

src/cli.rs

@@ -100,6 +100,13 @@ pub struct Cli {
     #[arg(help = crate::t!("Log level (error|warn|info|debug|trace). Also reads RUST_LOG.").into_owned(), long_help = None)]
     pub log: Option<String>,
 
+    /// Internal: probe one GPU backend offscreen (`dx12|vulkan|gl|sw`), print
+    /// one JSON line on success, then exit 0/1. Spawned by the GPU backend
+    /// resolver so a legacy-driver abort kills only the probe child, never
+    /// the editor itself.
+    #[arg(long, value_name = "BACKEND", hide = true)]
+    pub gpu_probe: Option<String>,
+
     /// Internal: run as the plugin runner child process.
     #[arg(long, value_names = ["SOCKET", "CDYLIB"], num_args = 2, hide = true)]
     pub ocs_plugin_runner: Option<Vec<String>>,
@@ -124,6 +131,12 @@ pub struct GuiConfig {
     pub compat_renderer: bool,
     /// Command lines to run once the editor is up.
     pub script_lines: Vec<String>,
+    /// Why this launch switched graphics backend, if it did. Shown once
+    /// as a command-line warning so the switch is never silent.
+    pub gpu_fallback_notice: Option<String>,
+    /// The GPU probe found no shader storage buffers: the packed
+    /// compatibility renderer was enabled automatically for this session.
+    pub gpu_compat_auto: bool,
 }
 
 /// Set once by `main` before the GUI boots; read by `app::boot`.