perf(messages): restructure format-string call sites (#2437)

## Summary

Phase 3 PR B of the message-interpolation cleanup. Handles the multi-line restructure sites flagged in `dev-docs/string-handling-message-interp-audit.md` (Phase 2). Phase 3.1 (PR #2436) handled trivial sweeps; this PR handles sites that needed an `if/else` hoist or switch restructure to eliminate `string.Format` while preserving exact message text.

Each site previously allocated an intermediate `string.Format(...)` result before passing to the message handler, despite Phase 1 making the handler accept interpolated string handlers natively.

## Sites fixed

- **`Projects/Server/Mobiles/Mobile.cs:7911`** - Title/guild header was using `string.Format` with a conditional template (`"[{1}]{2}"` vs `"[{0}, {1}]{2}"`). Split into `if (title.Length <= 0)` / `else` with direct `$"..."` interpolation.
- **`Projects/UOContent/Engines/ConPVP/DuelContext.cs:1337`** - View-ladder rank text used `string.Format(text, from == pm ? "You" : "They")`. Split into `if (from == pm)` / `else` with direct `$"..."` interpolation in each branch.
- **`Projects/UOContent/Engines/ConPVP/DuelContext.cs:1463`** - Showladder text reused a single format string for both `LocalOverheadMessage` ("You ... are ...") and `NonlocalOverheadMessage` ("`{pm.Name}` ... is ..."). Each call now uses an inline `$"..."` directly; no shared template.
- **`Projects/UOContent/Engines/ConPVP/Gumps/ConfirmSignupGump.cs:518`** - The signup confirmation message used a `switch` expression assigning a literal format string to `fmt`, then `string.Format(fmt, from.Female ? "Lady" : "Lord", timeUntil)`. Converted to a `switch` statement where each case calls `_registrar.PrivateOverheadMessage(...)` directly with an inline `$"..."`. Lady/Lord branching is hoisted to a `title` local.

## Exempted

- **`Projects/UOContent/Engines/ConPVP/Participant.cs:138`** - The `nonLocalOverhead` format string is a parameter passed in by callers of `Participant.Broadcast`. Investigation found 5 call sites in `DuelContext.cs` (lines 782, 802, 1187, 1196, and three at 1535/1564/1608) that pass distinct literal format strings. Refactoring would require changing all 5 callers and the method signature - out of scope for this PR. Marked with a `// Phase 3 audit:` comment per the audit's exemption convention.

Projects/Server/Mobiles/Mobile.cs

@@ -7908,14 +7908,26 @@ public partial class Mobile : IHued, IComparable<Mobile>, ISpawnable, IObjectPro
                     type = "";
                 }
 
-                var text = string.Format(
-                    title.Length <= 0 ? "[{1}]{2}" : "[{0}, {1}]{2}",
-                    title,
-                    guild.Abbreviation,
-                    type
-                );
-
-                PrivateOverheadMessage(MessageType.Regular, SpeechHue, true, text, from.NetState);
+                if (title.Length <= 0)
+                {
+                    PrivateOverheadMessage(
+                        MessageType.Regular,
+                        SpeechHue,
+                        true,
+                        $"[{guild.Abbreviation}]{type}",
+                        from.NetState
+                    );
+                }
+                else
+                {
+                    PrivateOverheadMessage(
+                        MessageType.Regular,
+                        SpeechHue,
+                        true,
+                        $"[{title}, {guild.Abbreviation}]{type}",
+                        from.NetState
+                    );
+                }
             }
         }