feat(udpscope): build scaffold and min/max envelope decimation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Client/udpscope/Decimate.cpp

@@ -0,0 +1,47 @@
+#include "Decimate.h"
+
+#include <algorithm>
+
+namespace udpscope {
+
+void MinMaxDecimate(const double* t, const double* v, size_t n,
+                    size_t maxPoints, Series& out) {
+    out.clear();
+    if (n == 0 || t == nullptr || v == nullptr) {
+        return;
+    }
+    if (n <= maxPoints || maxPoints < 4) {
+        out.t.assign(t, t + n);
+        out.v.assign(v, v + n);
+        return;
+    }
+
+    /* Two points per bucket, so the bucket count is half the budget. */
+    const size_t buckets = maxPoints / 2;
+    out.t.reserve(buckets * 2);
+    out.v.reserve(buckets * 2);
+
+    for (size_t b = 0; b < buckets; b++) {
+        const size_t begin = (n * b) / buckets;
+        size_t       end   = (n * (b + 1)) / buckets;
+        if (end <= begin) { end = begin + 1; }
+        if (end > n) { end = n; }
+
+        size_t lo = begin, hi = begin;
+        for (size_t i = begin + 1; i < end; i++) {
+            if (v[i] < v[lo]) { lo = i; }
+            if (v[i] > v[hi]) { hi = i; }
+        }
+
+        const size_t first  = std::min(lo, hi);
+        const size_t second = std::max(lo, hi);
+        out.t.push_back(t[first]);
+        out.v.push_back(v[first]);
+        if (second != first) {
+            out.t.push_back(t[second]);
+            out.v.push_back(v[second]);
+        }
+    }
+}
+
+} /* namespace udpscope */