fix: audit fixes — double-unlock guard, PAM OOM code, GPU blur, async fp stop (v0.5.1)

Security: prevent double unlock() when PAM and fingerprint succeed
simultaneously (ext-session-lock protocol error). Fix PAM callback
returning PAM_AUTH_ERR instead of PAM_BUF_ERR on calloc OOM.

Performance: replace CPU-side Gaussian blur (image crate) with GPU blur
via GskBlurNode + GskRenderer::render_texture(). Eliminates 500ms-2s
main-thread blocking on cold cache for 4K wallpapers. Remove image and
dirs dependencies (~15 transitive crates). Make fingerprint stop()
fire-and-forget async to avoid 6s UI block after successful auth.

DECISIONS.md

@@ -2,7 +2,14 @@
 
 Architectural and design decisions for Moonlock, in reverse chronological order.
 
-## 2026-03-28 – Optional background blur via `image` crate
+## 2026-03-28 – GPU blur via GskBlurNode replaces CPU blur
+
+- **Who**: Nyx, Dom
+- **Why**: CPU-side Gaussian blur (`image` crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache mitigated repeat starts but added ~100 lines of complexity.
+- **Tradeoffs**: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper. Removes `image` and `dirs` dependencies entirely. No disk cache needed.
+- **How**: `Snapshot::push_blur()` + `GskRenderer::render_texture()` on `connect_realize`. Blur happens once on the GPU when the widget gets its renderer, producing a concrete `gdk::Texture`. Zero startup latency.
+
+## 2026-03-28 – Optional background blur via `image` crate (superseded)
 
 - **Who**: Nyx, Dom
 - **Why**: Consistent with moonset/moongreet — blurred wallpaper as lockscreen background is a common UX pattern