feat: serial:/size: external matching, exclude Hyprland FALLBACK output

- external list now accepts serial: and size: prefixes; desc: matches
  serial too. Fixes docked-mode deadlock where Hyprland's synthetic
  FALLBACK output (created when the built-in is disabled by a stale
  docked config) was treated as a real external monitor, leaving the
  daemon stuck in docked after undocking.
- modes name resolution gains serial: prefix (resolveSerial).
- add -log-level flag for observability.
- add unit tests for external matching and name resolution.
- update README and example config.

monitor-lets-go.example.yaml

@@ -25,7 +25,10 @@ restore_on_exit: true
 
 # External monitors that trigger docked mode.
 #   Plain name: matches the connector name (e.g. DP-1, HDMI-A-1).
-#   desc: prefix: matches by monitor description (survives rename).
+#   desc: prefix: matches by monitor description or serial (survives rename).
+#   serial: prefix: matches by serial number (e.g. serial:3342300033911).
+#   size: prefix: matches by resolution, optionally with refresh
+#         (e.g. size:2560x1440 or size:2560x1440@165).
 # Optional — if omitted or left empty, the daemon auto-detects external
 # monitors: any display whose connector is not eDP-/LVDS-/DSI- is treated
 # as external.
@@ -33,13 +36,16 @@ external:
   - DP-1
   - DP-2
   - desc:Dell Inc. DELL U2723QE
+  - serial:3342300033911
+  - size:3440x1440@120
 
 # Monitor layouts for each mode.
 # "portable" and "docked" are required.
 # Monitors connected but not listed in the mode are automatically disabled.
 #
 # Fields:
-#   name     — connector name, desc:description, size:WxH, or size:WxH@R
+#   name     — connector name, desc:description, serial:XXXX, size:WxH,
+#              or size:WxH@R
 #   enabled  — true to show, false to disable
 #   mode     — "preferred" (auto-detect), "1920x1080@60", etc.
 #   position — "auto", "0x0", "1920x0", etc.