Fix 5 deferred performance/UX issues from audit

M8: Pre-compile regex once before rename preview loop instead of
recompiling per file. Adds apply_simple_compiled() to RenameConfig.

M9: Cache font data in watermark module using OnceLock (default font)
and Mutex<HashMap> (named fonts) to avoid repeated filesystem walks
during preview updates.

M12: Add 150ms debounce to watermark opacity, rotation, margin, and
scale sliders to avoid spawning preview threads on every pixel of
slider movement.

M13: Add 150ms debounce to compress per-format quality sliders (JPEG,
PNG, WebP, AVIF) for the same reason.

M14: Move thumbnail loading to background threads instead of blocking
the GTK main loop. Each thumbnail is decoded via image crate in a
spawned thread and delivered to the main thread via channel polling.

pixstrip-core/src/operations/mod.rs

@@ -318,6 +318,11 @@ pub struct RenameConfig {
 fn default_counter_position() -> u32 { 3 }
 
 impl RenameConfig {
+    /// Pre-compile the regex for batch use. Call once before a loop of apply_simple_compiled.
+    pub fn compile_regex(&self) -> Option<regex::Regex> {
+        rename::compile_rename_regex(&self.regex_find)
+    }
+
     pub fn apply_simple(&self, original_name: &str, extension: &str, index: u32) -> String {
         // 1. Apply regex find-and-replace on the original name
         let working_name = rename::apply_regex_replace(original_name, &self.regex_find, &self.regex_replace);
@@ -378,4 +383,68 @@ impl RenameConfig {
 
         format!("{}.{}", result, extension)
     }
+
+    /// Like apply_simple but uses a pre-compiled regex (avoids recompiling per file).
+    pub fn apply_simple_compiled(&self, original_name: &str, extension: &str, index: u32, compiled_re: Option<&regex::Regex>) -> String {
+        // 1. Apply regex find-and-replace on the original name
+        let working_name = match compiled_re {
+            Some(re) => rename::apply_regex_replace_compiled(original_name, re, &self.regex_replace),
+            None => original_name.to_string(),
+        };
+
+        // 2. Apply space replacement
+        let working_name = rename::apply_space_replacement(&working_name, self.replace_spaces);
+
+        // 3. Apply special character filtering
+        let working_name = rename::apply_special_chars(&working_name, self.special_chars);
+
+        // 4. Build counter string
+        let counter_str = if self.counter_enabled {
+            let counter = self.counter_start.saturating_add(index.saturating_sub(1));
+            let padding = (self.counter_padding as usize).min(10);
+            format!("{:0>width$}", counter, width = padding)
+        } else {
+            String::new()
+        };
+
+        let has_counter = self.counter_enabled && !counter_str.is_empty();
+
+        // 5. Assemble parts based on counter position
+        let mut result = String::new();
+
+        if has_counter && self.counter_position == 0 {
+            result.push_str(&counter_str);
+            result.push('_');
+        }
+
+        result.push_str(&self.prefix);
+
+        if has_counter && self.counter_position == 1 {
+            result.push_str(&counter_str);
+            result.push('_');
+        }
+
+        if has_counter && self.counter_position == 4 {
+            result.push_str(&counter_str);
+        } else {
+            result.push_str(&working_name);
+        }
+
+        if has_counter && self.counter_position == 2 {
+            result.push('_');
+            result.push_str(&counter_str);
+        }
+
+        result.push_str(&self.suffix);
+
+        if has_counter && self.counter_position == 3 {
+            result.push('_');
+            result.push_str(&counter_str);
+        }
+
+        // 6. Apply case conversion
+        let result = rename::apply_case_conversion(&result, self.case_mode);
+
+        format!("{}.{}", result, extension)
+    }
 }