feat(displays): hotplug auto-switch for display profiles (item 15b)

nomarchy-display-profile gains `match <outputs>`: an exact set match
wins, else the largest profile whose named outputs are all connected;
any tie prints nothing — the watcher must never flap between ambiguous
layouts. nomarchy-display-profile-watch (exec-once when profiles are
declared, the keyboard-watcher poll pattern — session units race
Hyprland IPC on relogin) reacts to output-set CHANGES only, reads
settings.displayProfileAuto from the live state each time (the menu's
new Auto-switch row toggles it instantly, no rebuild), and applies via
the tool — so an auto-pick persists a concrete profile that the next
rebuild bakes. A separate boolean rather than an "auto" pseudo-profile
for exactly that bake semantics.

Verified: V0 (flake check); V1 — a scratch 4-profile downstream builds;
tool + watcher pass bash -n; the match matrix exercised end-to-end:
three exact picks, an unambiguous-superset pick, a tie -> none, a
no-fit -> none; the watcher is exec-once'd in the rendered config; the
menu renders the Auto-switch row. Remains: V3 real dock/undock
(HARDWARE-QUEUE, rides the slice-a entry). Slice c (workspace binding)
stays, tagged optional.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/downstream/home.nix

@@ -42,7 +42,9 @@
   #   ];
   #   undocked = [ { name = "eDP-1"; position = "0x0"; } ];
   # };                                  #   applied instantly + remembered in the
-  #                                     #   flake state (settings.displayProfile)
+  #                                     #   flake state (settings.displayProfile);
+  #                                     #   the menu's Auto-switch row makes dock/
+  #                                     #   undock pick the matching profile
   #
   # nomarchy.keyboard.devices = {       # a specific keyboard's own layout
   #   "keychron-keychron-k2" = { layout = "de"; };