refactor: convert to Cargo workspace, move gtklock crate

Replaces the single-crate Cargo.toml with a workspace containing
ahfail-gtklock (migrated from root src/) and three stub crates
(ahfail-ui, ahfail-pam, ahfail-display). Updates meson.build to
build with -p ahfail-gtklock.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -3,7 +3,11 @@
 version = 4
 
 [[package]]
-name = "ahfail"
+name = "ahfail-display"
+version = "0.1.0"
+
+[[package]]
+name = "ahfail-gtklock"
 version = "0.1.0"
 dependencies = [
  "gdk",
@@ -14,11 +18,18 @@ dependencies = [
  "gstreamer-player",
  "gtk",
  "libc",
- "once_cell",
  "pkg-config",
  "rand",
 ]
 
+[[package]]
+name = "ahfail-pam"
+version = "0.1.0"
+
+[[package]]
+name = "ahfail-ui"
+version = "0.1.0"
+
 [[package]]
 name = "anyhow"
 version = "1.0.100"