feat: SG Filter on/off 토글 추가 (Settings 패널)

- PiezoEchoAnalyzer.useSgFilter: true/false로 SG 필터 on/off
- OFF 시 raw ADC 그대로 low-echo detection에 사용
- Settings 패널에 Switch 토글 추가 (기본 ON)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/java/com/example/medilightv2android/managers/PiezoEchoAnalyzer.kt

@@ -213,9 +213,10 @@ class PiezoEchoAnalyzer private constructor() {
 
     // ── Denoising Pipeline ──
 
-    /** SG 디노이징 (6ch 알고리즘: TVD 제거, SG만 사용) */
+    var useSgFilter: Boolean = true
+
     fun denoise(signal: DoubleArray): DoubleArray {
-        return sgSmooth(signal)
+        return if (useSgFilter) sgSmooth(signal) else signal.copyOf()
     }
 
     /** 1D Total Variation Denoising — Chambolle (2004) dual projected gradient */