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

src/app.rs

@@ -587,14 +587,10 @@ fn Hero(title: String, description: String, landing: bool, site: SiteConfig, cur
         None
     };
     let has_module = module.is_some();
-    // Light theme inverts white-stroke wordmarks to ink. The house
-    // mark and the sibling-site marks are all white-stroke SVGs; a
-    // raster logo is a client brand and keeps its colors.
-    let house_wordmark = site
-        .wordmark
-        .as_deref()
-        .map(|w| w.ends_with(".svg"))
-        .unwrap_or(true);
+    // Light theme can invert a white-stroke wordmark to ink. Default:
+    // only the built-in house mark inverts; a content-shipped logo
+    // keeps its colours unless site.yaml opts in with wordmark_invert.
+    let house_wordmark = site.wordmark_invert.unwrap_or(site.wordmark.is_none());
     let wordmark = site.wordmark.clone().unwrap_or_else(|| "/wordmark.svg".to_string());
     // Clamped server-side too, but belt and braces for the inline style.
     let wordmark_style = site