实现市值区间padding机制

- 添加padding_ratio、min_padding、max_padding配置参数
- 在市值区间计算中应用padding扩大选股范围
- 更新OmniMicroCapConfig、CnSmallCapRotationConfig和DynamicMarketCapBandSelector
- AiQuant V1.0.4默认padding: ratio=0.5, min=12.5, max=30.0
- 目标：增加候选股票数量，匹配AiQuant行为

crates/fidc-core/src/universe.rs

@@ -78,6 +78,9 @@ pub struct DynamicMarketCapBandSelector {
     pub cap_span: f64,
     pub xs: f64,
     pub top_n: usize,
+    pub padding_ratio: f64,
+    pub min_padding: f64,
+    pub max_padding: f64,
 }
 
 impl DynamicMarketCapBandSelector {
@@ -87,6 +90,9 @@ impl DynamicMarketCapBandSelector {
         cap_span: f64,
         xs: f64,
         top_n: usize,
+        padding_ratio: f64,
+        min_padding: f64,
+        max_padding: f64,
     ) -> Self {
         Self {
             base_index_level,
@@ -94,11 +100,14 @@ impl DynamicMarketCapBandSelector {
             cap_span,
             xs,
             top_n,
+            padding_ratio,
+            min_padding,
+            max_padding,
         }
     }
 
     pub fn demo(top_n: usize) -> Self {
-        Self::new(2000.0, 7.0, 10.0, 4.0 / 500.0, top_n)
+        Self::new(2000.0, 7.0, 10.0, 4.0 / 500.0, top_n, 0.5, 8.0, 20.0)
     }
 
     pub fn regime(&self, benchmark_level: f64) -> BandRegime {
@@ -114,7 +123,18 @@ impl DynamicMarketCapBandSelector {
     pub fn band_for_level(&self, benchmark_level: f64) -> (f64, f64) {
         let start = ((benchmark_level - self.base_index_level) * self.xs) + self.base_cap_floor;
         let low = start.round();
-        (low, low + self.cap_span)
+        let high = low + self.cap_span;
+        
+        // Apply padding to expand the range
+        let span = high - low;
+        let padding = (span * self.padding_ratio)
+            .max(self.min_padding)
+            .min(self.max_padding);
+        
+        let lower_bound = (low - padding).max(0.0);
+        let upper_bound = high + padding;
+        
+        (lower_bound, upper_bound)
     }
 }