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.

README.md

@@ -144,11 +144,15 @@ restore_on_exit: true
 
 # External monitors that trigger docked mode.
 # Plain name: matches connector (DP-1, HDMI-A-1).
-# desc: prefix: matches by monitor description (survives port rename).
+# desc: prefix: matches by monitor description or serial (survives port rename).
+# serial: prefix: matches by serial number.
+# size: prefix: matches by resolution, optionally with refresh rate (size:2560x1440@165).
 # Optional — if omitted or empty, the daemon auto-detects external monitors:
 # any display whose connector is not eDP-/LVDS-/DSI- is treated as external.
 external:  # optional
   - desc:Dell Inc. DELL U2723QE
+  - serial:3342300033911
+  - size:3440x1440
   - DP-9
   - DP-10
 
@@ -197,19 +201,22 @@ hooks:
 
 ### Monitor matching
 
-Three match modes for the `modes` section (the `external` list supports plain names and `desc:`):
+Match modes for the `modes` section and the `external` list. The `external` list supports all four modes below (plus auto-detect when empty); the `modes` section supports all four as `name:` values:
 
 | Syntax | Matches | Use case |
 |---|---|---|
 | `DP-1` | Exact connector name | Simple setups, built-in displays |
 | `desc:Dell U2723QE` | Substring in monitor description or serial | Survives port rename across different docks |
-| `size:2560x1440` | Exact pixel dimensions | Two identical monitors with same resolution |
+| `serial:3342300033911` | Substring in serial number | Uniquely identifies a specific monitor |
+| `size:2560x1440` | Exact pixel dimensions | Match by resolution instead of connector name |
 | `size:2560x1440@165` | Dimensions + refresh rate | Disambiguate identical models |
 
 Sway description format: `make model serial_widthxheight` (with serial omitted if `Unknown`).  
-Run `hyprctl monitors all` (Hyprland) or `swaymsg -t get_outputs` (Sway) to see your monitor names and descriptions.
+Run `hyprctl monitors all` (Hyprland) or `swaymsg -t get_outputs` (Sway) to see your monitor names, descriptions, and serials.
 
-**desc: and size: in modes** — when a monitor name in `modes` uses `desc:` or `size:`, the daemon resolves it to the actual connector name at runtime. Ambiguous matches (a prefix matching multiple monitors with identical resolution) cause an error. For `size:`, add `@R` (refresh rate) to disambiguate monitors with the same resolution. Unresolvable names also cause an error.
+**desc:, serial: and size: in modes** — when a monitor name in `modes` uses `desc:`, `serial:`, or `size:`, the daemon resolves it to the actual connector name at runtime. Ambiguous matches (a prefix matching multiple monitors) cause an error. For `size:`, add `@R` (refresh rate) to disambiguate monitors with the same resolution. Unresolvable names also cause an error.
+
+**External list** — entries are checked against every detected monitor; a single match triggers docked mode. When the list is empty, any non-internal connector is treated as external.
 
 ### Backend-specific configuration
 
@@ -351,20 +358,23 @@ Check the hook command works from a terminal first. Hooks run via `sh -c`, so sh
 
 ### Monitor names changed after reboot
 
-Use `desc:` or `size:` prefix matching instead of connector names. Both survive port renames across different docks and reboots.
+Use `desc:`, `serial:`, or `size:` prefix matching instead of connector names. All three survive port renames across different docks and reboots.
 
-**Hyprland:** `hyprctl monitors all` to see descriptions.  
+**Hyprland:** `hyprctl monitors all` to see descriptions and serials.  
 **Sway:** `swaymsg -t get_outputs` to see names, make/model, serial, and native resolution.
 
 ```yaml
 external:
   - desc:Dell Inc. DELL U2723QE   # survives port rename
+  - serial:3342300033911          # match a specific monitor by serial
 
 modes:
   docked:
     monitors:
       - name: desc:Dell Inc. DELL U2723QE   # also works here
         enabled: true
+      - name: serial:3342300033911          # and here
+        enabled: true
       - name: size:2560x1440@165            # match by dimensions + refresh
         enabled: true
 ```