perf(cli): avoid heap allocation in bias scorer spam check

cli/benches/quality_bench.rs

@@ -1,4 +1,5 @@
 use criterion::{Criterion, criterion_group, criterion_main};
+use do_wdr_lib::bias_scorer::score_result;
 use do_wdr_lib::quality::score_content;
 use std::hint::black_box;
 
@@ -53,5 +54,39 @@ One more subscribe button.
     });
 }
 
-criterion_group!(benches, bench_quality_scoring);
+fn bench_bias_scoring(c: &mut Criterion) {
+    let content = r#"
+# Sample Page
+
+This is a sample page with some content.
+It has multiple lines.
+
+Cookie Policy: We use cookies.
+All rights reserved (c) 2026.
+Privacy Policy is available here.
+
+```rust
+fn main() {
+    println!("Hello, world!");
+}
+```
+
+Subscribe to our newsletter for more updates.
+Follow us on Twitter.
+Click here to learn more.
+buy now cheap discount free trial best price
+"#
+    .repeat(10);
+
+    c.bench_function("score_result", |b| {
+        b.iter(|| {
+            score_result(
+                black_box("https://github.com/rust-lang/rust"),
+                black_box(&content),
+            );
+        });
+    });
+}
+
+criterion_group!(benches, bench_quality_scoring, bench_bias_scoring);
 criterion_main!(benches);

cli/src/bias_scorer.rs

@@ -2,6 +2,21 @@
 
 use url::Url;
 
+/// Check if a string contains another string case-insensitively (ASCII-only)
+fn contains_ignore_ascii_case(haystack: &str, needle: &str) -> bool {
+    if needle.is_empty() {
+        return true;
+    }
+    let haystack_bytes = haystack.as_bytes();
+    let needle_bytes = needle.as_bytes();
+    if haystack_bytes.len() < needle_bytes.len() {
+        return false;
+    }
+    haystack_bytes
+        .windows(needle_bytes.len())
+        .any(|window| window.eq_ignore_ascii_case(needle_bytes))
+}
+
 /// Score a result based on domain trust and content quality
 pub fn score_result(url: &str, content: &str) -> f64 {
     let mut score: f64 = 0.5;
@@ -53,11 +68,10 @@ pub fn score_result(url: &str, content: &str) -> f64 {
         score += 0.05;
     }
 
-    // SEO spam detection
+    // SEO spam detection using zero-allocation case-insensitive ASCII substring search
     let spam_terms = ["buy now", "cheap", "discount", "free trial", "best price"];
-    let lower_content = content.to_lowercase();
     for term in spam_terms {
-        if lower_content.contains(term) {
+        if contains_ignore_ascii_case(content, term) {
             score -= 0.1;
         }
     }