fix(markdown): restore ratatui bold assertions

Acceptance-Criteria:
- cargo test -p owlen-markdown completes without Style::contains usage
- Workspace lint hook passes under cargo clippy --all-features -D warnings
- Markdown heading and inline code tests still confirm bold styling

Test-Notes:
- cargo test -p owlen-markdown
- cargo clippy -p owlen-markdown --tests -- -D warnings
- cargo clippy --all-features -- -D warnings

crates/owlen-core/src/usage.rs

@@ -167,10 +167,7 @@ impl UsageLedger {
             Err(_) => 0,
         };
 
-        let entry = self
-            .providers
-            .entry(provider.to_string())
-            .or_insert_with(VecDeque::new);
+        let entry = self.providers.entry(provider.to_string()).or_default();
 
         entry.push_back(UsageRecord {
             timestamp: ts,