feat(chat): CT-C2 Escape leaves the entry; CT-C3 the timestamp is grey

Campaign CT slices C2 and C3.

**C2 — Escape in the chat input did nothing at all.** Not "did the wrong
thing": nothing. Two independent facts had to hold for that. UiField has no
Escape case, AND a focused field reports IsEditControl, which makes UiRoot skip
its own fallback and the input dispatcher withhold game actions — so the player
had no way out of the bar except the mouse.

Retail maps Escape to input action 0x0B, which runs
ChatInterface::DeactivateChatEntry @0x004F2FC0: RelinquishFocus, then
Deactivate. It does NOT clear the field. That is worth stating because the
obvious guess — "Escape clears the input" — is wrong and would have looked
perfectly reasonable; a half-written message survives stepping away from the
bar, and the test pins that rather than just pinning "handled".

**C3 — the timestamp took the message's colour.** Retail appends it as its own
run at a FIXED colour index (0x0C, which BuildChatColorLookupTable @0x004F31C0
fills with colorGrey) rather than the line's, so it stays grey whether the
message is red combat text or white speech.

Most of C3 was already done and stayed untouched: the DisplayTimeStamps option
is polled, and FormatTimestampPrefix already matches retail's "%#H:%M:%S ".
Only the colour was wrong, and it was only fixable now because A1/A4 made a
line able to carry more than one colour.

The stamp is a span ROLE rather than a second tag type: it is not clickable and
carries no payload, so modelling it as a tag would have made it hit-testable
for no reason. Its colour comes from the same runtime table every message
colour comes from, unlike the tagged-name colour, which is authored per element
(0x1D) and deliberately lives elsewhere.

One consequence worth naming: a timestamped line now needs runs even when its
sender is not tagged, because the stamp alone is reason enough. Before this,
only tagged lines got runs.

Also verified and NOT changed, having checked rather than assumed: C1's
auto-scroll half is already retail-faithful — UiScrollable.SetExtents samples
"was at the end" BEFORE applying new extents and only re-sticks if so, which is
exactly retail's IsAtVerticalEnd rule, and chat gets it by default. C1 reduces
to the unread indicator, which does not exist yet.

Solution builds clean; full hermetic gate green.

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

src/AcDream.Core/Chat/ChatTagMarkup.cs

@@ -51,8 +51,26 @@ public readonly record struct ChatTextTag(string Type, string Format, string Dat
     }
 }
 
+/// <summary>What a span IS, for colouring purposes.</summary>
+public enum ChatSpanRole
+{
+    /// <summary>Ordinary message text; takes the line's own colour.</summary>
+    Body,
+
+    /// <summary>
+    /// The leading timestamp. Retail appends it as its own run at a FIXED
+    /// colour index (<c>0x0C</c>) rather than the line's
+    /// (<c>ChatInterface::RecvNotice_DisplayFinalStringInfo @0x004F4640</c>),
+    /// so it stays grey whatever colour the message is.
+    /// </summary>
+    Timestamp,
+}
+
 /// <summary>One stretch of chat text, and the tag covering it (if any).</summary>
-public readonly record struct ChatTextSpan(string Text, ChatTextTag? Tag);
+public readonly record struct ChatTextSpan(
+    string Text,
+    ChatTextTag? Tag,
+    ChatSpanRole Role = ChatSpanRole.Body);
 
 /// <summary>
 /// Parses retail's inline chat tag markup into spans.