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/content.rs

@@ -523,6 +523,12 @@ pub struct SiteConfig {
     /// `None` falls back to `/wordmark.svg`.
     #[serde(default)]
     pub wordmark: Option<String>,
+    /// Whether to invert the wordmark in light theme. `None` inverts
+    /// only the built-in house wordmark (a white-stroke SVG); a
+    /// content-shipped logo keeps its own colours unless it sets this
+    /// true (e.g. a sibling site's white-stroke mark).
+    #[serde(default)]
+    pub wordmark_invert: Option<bool>,
     /// Wordmark display height in rem (0.5–6.0). `None` keeps the
     /// stylesheet's default. Raster logos look best at or below their
     /// intrinsic pixel height.