Add per-profile "Lock to exact peer addresses" setting (#17, held for next release)

Feature request from the same singer: a way to pin a profile to exact peer addresses
so RemSound never matches the other computer by its advertised name and never switches
to a different address it discovers — for a machine reachable at two addresses at once
(VPN + LAN), they want only the one they chose, and would rather the connection die than
wander. The logical end of the v4.7/v4.8 direction.

New per-profile Profile.LockPeerAddresses (default false), round-tripped through
RemSoundSettingsStore (ApplyProfile/CopyTo + Load/SaveLockPeerAddresses), mirroring the
PriorityMode pattern. New AccessibleCheckBox on the Connectivity tab ("Lock to these exact
peer addresses, no matter what — never follow names or switch", Alt+L), saved with the
profile, marks the profile dirty on change. When set, RefreshKnownPeers early-returns
before the discovered-peer merge and the address-follow, so the profile's peers stay
exactly as set (the allow-list is still pushed). Off = unchanged behaviour.

Manual: rewrote the "Connecting to one specific IP" section (which over-promised that
add-by-IP "can never drift" — the merge/follow could) into an unambiguous "Locking a
profile to one exact address" section, plus a Connectivity-tab control-table row.

Held for the next release.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.Core/Profile.cs

@@ -177,6 +177,12 @@ public sealed class Profile
     /// list at save time. On load, RemSound auto-connects to any of these that resolve.</summary>
     public List<string> SelectedConnectedPeers { get; set; } = [];
 
+    /// <summary>When true, this profile is LOCKED to the exact peer addresses set above. RemSound uses
+    /// only those addresses, never matches the other computer by the name it advertises on the network,
+    /// and never switches to a different address it discovers — even if the set address stops working
+    /// (the connection simply waits or drops rather than moving). Off by default. (#17)</summary>
+    public bool LockPeerAddresses { get; set; }
+
     // === Hotkeys ===
     public HotkeyRecord? ReceiveMuteHotkey { get; set; }
     public HotkeyRecord? SendMuteHotkey { get; set; }