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- gpu-tuning/plan.md gets a superseded note rather than rewriting the original rationale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.

docs/gpu-tuning/plan.md

@@ -43,7 +43,7 @@ Metal / CUDA counterparts slot in when those backends grow in Magnotia. Today Ma
 - New `SettingsState.gpuTuning: { disableCoopmat: boolean, forceFp32: boolean, disableF16: boolean, disableIntegerDotProduct: boolean, enableValidation: boolean }` in [src/lib/types/app.ts](../../src/lib/types/app.ts)
 - All defaults `false` in [src/lib/stores/page.svelte.ts](../../src/lib/stores/page.svelte.ts)
 - Persistence uses the existing `save_preferences` → SQLite `magnotia_preferences` path
-- Backend reads preferences at the **very top** of `run()` in [src-tauri/src/lib.rs](../../src-tauri/src/lib.rs) — before `tauri::Builder::default()` spawns threads — and writes via `unsafe { std::env::set_var(...) }`. Matches the existing `ensure_x11_on_wayland` pattern
+- Backend process env must be established before GPU backend initialisation. **Superseded note (2026-05-12):** runtime `std::env::set_var` mutation and the old `ensure_x11_on_wayland` pattern were removed from `src-tauri/src/lib.rs`; launcher/wrapper processes now own env-var setup. Preserve this startup-order requirement when revisiting GPU tuning, but do not reintroduce in-process env mutation.
 - Settings UI shows a sticky "Restart required for changes to take effect" banner when any toggle has drifted from its launch-time value
 - A "Reset to defaults" button zeroes all toggles