agent: dev launcher — own Linux env-var contract, add dev:tauri, doc sweep

The 2026-05-12 engine-slop pass removed runtime std::env::set_var mutation from src-tauri/src/lib.rs and replaced it with warn_if_x11_env_unset_on_wayland. With no launcher owning the contract, Linux Wayland dogfood was about to regress.

run.sh:
- now owns Linux launcher env defaults via case "$(uname -s)"
  LIBCLANG_PATH=/usr/lib64/llvm21/lib64 (user-set wins)
  WEBKIT_DISABLE_DMABUF_RENDERER=1 (always on Linux; user-set wins)
  GDK_BACKEND=x11, WINIT_UNIX_BACKEND=x11 (Wayland only; user-set wins)
- 60s Vite readiness timeout
- detects early Vite exit (kill -0 + wait) instead of hanging forever
- trap installed before wait so Ctrl-C cleans up
- args forwarded: ./run.sh --release etc.
- non-exec final Tauri launch preserved so cleanup trap fires

package.json:
- "dev:tauri": "./run.sh" — canonical discoverable dev command

Docs:
- README, dev-setup, architecture-map runtime + launcher pages updated with the new contract; canonical command is npm run dev:tauri (./run.sh as direct equivalent)
- dev-launcher-and-scripts.md replaces the incorrect "kills process group" claim with honest "kills the spawned npm process"
- dev-setup.md path /CORBEL-Projects/magnotia → /CORBEL-Projects/transcription-app
- gpu-tuning/plan.md gets a superseded note rather than rewriting the original rationale
- engine-slop-residuals.md gains Area F (packaged-binary launcher contract) with honest wrapper-vs-.desktop trade-off documented

Verification: bash -n run.sh; shellcheck clean; cargo check -p magnotia green; npm pkg get 'scripts.dev:tauri' returns "./run.sh"; stale-ref sweep clean across living docs.

src-tauri/src/lib.rs

@@ -103,19 +103,18 @@ async fn save_preferences(
 }
 
 /// On Linux Wayland sessions (KDE, GNOME) WebKitGTK + DMABUF rendering hits
-/// known crashes that the HANDOVER documents working around with a manual
-/// env-var prefix:
+/// known crashes that the dev launcher works around by setting rendering
+/// env vars before WebKitGTK/WINIT initialise. In development, launch via:
 ///
 /// ```sh
-/// env GDK_BACKEND=x11 WINIT_UNIX_BACKEND=x11 \
-///     WEBKIT_DISABLE_DMABUF_RENDERER=1 npm run tauri dev
+/// npm run dev:tauri
 /// ```
 ///
 /// Detect the Wayland session at startup and warn when the process was not
 /// launched with the safe Linux rendering env vars. Rust 2024 marks runtime
 /// environment mutation unsafe in multi-threaded programs, so the app must be
-/// launched with these values by the desktop file, package wrapper, or dev
-/// shell rather than patching them here.
+/// launched with these values by `run.sh`, the desktop file, or a package
+/// wrapper rather than patching them here.
 ///
 /// Inspired by Open-Whispr's similar Chromium self-relaunch (with
 /// --ozone-platform=x11). Day 6 of the upgrade plan.
@@ -139,7 +138,7 @@ fn warn_if_x11_env_unset_on_wayland() {
     // renderer for no visible quality difference. Empirically (env-var
     // matrix on Ryzen 5 4650U / Vega 6, May 2026) toggling this dropped
     // magnotia idle CPU 12.30% → 2.80% of one core and idle GPU 17% → 10%.
-    // Apples to both X11 and Wayland sessions; users can opt back in by
+    // Applies to both X11 and Wayland sessions; users can opt back in by
     // setting WEBKIT_DISABLE_DMABUF_RENDERER=0 explicitly.
     warn_if_unset(
         "WEBKIT_DISABLE_DMABUF_RENDERER",