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

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

Client/udpscope/Decimate.h

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+/**
+ * @file Decimate.h
+ * @brief Min/max envelope decimation for screen rendering.
+ */
+#pragma once
+
+#include "Types.h"
+
+namespace udpscope {
+
+/**
+ * @brief Reduce n points to at most maxPoints by emitting each bucket's
+ *        minimum and maximum, in time order.
+ *
+ * LTTB is deliberately not used. It selects representative points and will
+ * silently drop a one-sample glitch; on a scope that glitch is usually the
+ * thing being looked for. The emitted pair stays in time order rather than
+ * value order because callers binary-search the result by time.
+ *
+ * Input shorter than maxPoints is copied through unchanged.
+ */
+void MinMaxDecimate(const double* t, const double* v, size_t n,
+                    size_t maxPoints, Series& out);
+
+} /* namespace udpscope */