wordmark_invert: explicit, not inferred from .svg

A content-shipped colour logo (Klingenberg's red tile) was being
inverted to teal in light theme by the .svg heuristic. Invert is now
an explicit site.yaml flag defaulting to house-mark-only; content
logos keep their colours unless they opt in.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y42TyF8Zu7NGRR2893vNcZ

Cargo.lock

@@ -2948,7 +2948,7 @@ dependencies = [
 
 [[package]]
 name = "portal"
-version = "0.3.25"
+version = "0.3.26"
 dependencies = [
  "anyhow",
  "arc-swap",