docs(messages): document interpolation anti-patterns and :L format spec (#2441)

## Summary

Captures the durable learnings from the message-interpolation work (PRs #2434, #2436, #2437, #2438, #2440) as reference documentation. **Doc-only PR — no code changes.**

The original Phase 2 audit (PR #2435) was development scaffolding and was closed unmerged once Phase 3 consumed it. This PR replaces it with proper reference docs that future authors can consult.

## What's added

### `dev-docs/string-handling.md`
- Promote `RawInterpolatedStringHandler` from a one-line note to a proper section listing all APIs that accept it (messages, OPL, gumps, packets).
- Document the `:L` lowercase format specifier.
- New comprehensive **"Interpolation Anti-Patterns"** section covering 8 patterns with before/after examples — applies to any handler-aware API:
  1. Ternary with interpolated branches
  2. Switch expression with interpolated arms
  3. Pre-built local typed as `string`
  4. `.ToString()` (or any string-returning method) inside a hole
  5. String concatenation inside a hole
  6. `string.Format` feeding a handler-aware API
  7. LINQ-built strings inside a hole
  8. Pre-built concat var

### `dev-docs/networking-packets.md`
- Add **"Player-Facing Message APIs"** section listing `Mobile` / `Item` / `NetState` message methods with their handler overloads.
- Note the `IBroadcastFilter` pattern for new spatial-broadcast helpers.

### `dev-docs/property-lists.md`, `dev-docs/gump-system.md`
- Cross-reference the new anti-patterns section.
- Add explicit `.ToString()` inside holes warning to property-lists (it had no such guidance before).

### `dev-docs/claude-skills/`
- Mirror the same content (condensed) in `modernuo-string-handling.md`, `modernuo-networking.md`, `modernuo-property-lists.md`, `modernuo-gump-system.md`.
- Add audit rule #17 to `modernuo-code-audit.md` covering all 8 anti-patterns with severity WARNING, plus the `:L` format spec.

### `CLAUDE.md`
- Add audit rule #18 summarizing the interpolation anti-patterns + `:L`, pointing to `dev-docs/string-handling.md` for details.

## Why this matters

Before this PR there was no documentation explaining when an interpolated string call site silently allocates a string despite the receiving API providing a handler overload. The Phase 3 cleanup (PRs #2436/#2437/#2438) discovered ~28 such sites in the codebase; without these docs the same patterns would re-emerge. The new audit rule + CLAUDE.md entry will catch them at write time.

dev-docs/gump-system.md

@@ -200,6 +200,18 @@ builder.AddHtmlLocalized(x, y, w, h, clilocNumber);   // Localized text
 builder.AddHtmlLocalized(x, y, w, h, clilocNumber, color);
 ```
 
+#### Interpolation in text
+
+Most text-accepting builders take a `ReadOnlySpan<char>` and have a `ref RawInterpolatedStringHandler` overload, so `$"..."` literals at the call site are zero-allocation. The same applies to `Html.Center`, `Html.Color`, `Html.Right` helpers used when wrapping text in HTML markup:
+
+```csharp
+// Zero allocation — interpolation handler renders directly into a pooled buffer
+builder.AddHtml(20, 20, 200, 100, $"<center>{Title}: {Score:N0}</center>");
+builder.AddLabel(20, 40, hue, $"You have {gold} gold");
+```
+
+Several call-site shapes silently defeat the handler overload selection (ternaries with interpolated branches, `.ToString()` inside holes, pre-built `var msg = $"..."` locals, etc.). See [`dev-docs/string-handling.md`](string-handling.md#interpolation-anti-patterns) for the full list and fixes — they apply equally inside `BuildLayout`.
+
 ### Interactive Elements
 ```csharp
 builder.AddButton(x, y, normalID, pressedID, buttonID);