fix: audit fixes — D-Bus sender validation, fp lifecycle, multi-monitor caching (v0.6.0)

Close the only exploitable auth bypass: validate VerifyStatus signal sender
against fprintd's unique bus name. Fix fingerprint D-Bus lifecycle so devices
are properly released on verify-match and async restarts check the running
flag between awaits.

Security: num_msg guard in PAM callback, symlink rejection for background_path,
peek icon disabled, TOML parse errors logged, panic hook before logging.

Performance: blur and avatar textures cached across monitors, release profile
with LTO/strip.

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonlock, in reverse chronological order.
 
+## 2026-03-28 – Audit-driven security and lifecycle fixes (v0.6.0)
+
+- **Who**: Nyx, Dom
+- **Why**: Triple audit (quality, performance, security) revealed a critical D-Bus signal spoofing vector, fingerprint lifecycle bugs, and multi-monitor performance issues.
+- **Tradeoffs**: `cleanup_dbus()` extraction adds a method but clarifies the stop/match ownership; `running_flag: Rc<Cell<bool>>` adds a field but prevents race between async restart and stop; sender validation adds a check per signal but closes the only known auth bypass.
+- **How**: (1) Validate D-Bus VerifyStatus sender against fprintd's unique bus name. (2) Extract `cleanup_dbus()` from `stop()`, call it on verify-match. (3) `Rc<Cell<bool>>` running flag checked after await in `restart_verify_async`. (4) Consistent 3s D-Bus timeouts. (5) Panic hook before logging. (6) Blur and avatar caches shared across monitors. (7) Peek icon disabled. (8) Symlink rejection for background_path. (9) TOML parse errors logged.
+
 ## 2026-03-28 – GPU blur via GskBlurNode replaces CPU blur
 
 - **Who**: Nyx, Dom