feat(downstream): add cvmmap downstream runtime implementation

This commit introduces the full downstream runtime implementation needed to ingest, transform, and publish streams.

It preserves the original upstream request boundary by packaging the entire cvmmap-streamer module (build config, public API, protocol and IPC glue, and simulator/tester entrypoints) in one coherent core unit.

Keeping this group isolated enables reviewers to validate runtime behavior and correctness without mixing test evidence or process documentation changes.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

include/cvmmap_streamer/metrics/latency_tracker.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <cstdint>
+#include <deque>
+#include <vector>
+
+namespace cvmmap_streamer::metrics {
+
+struct LatencySummary {
+	std::uint64_t samples{0};
+	std::uint64_t min_us{0};
+	std::uint64_t max_us{0};
+	std::uint64_t avg_us{0};
+	std::uint64_t p50_us{0};
+	std::uint64_t p95_us{0};
+	std::uint64_t p99_us{0};
+};
+
+class IngestEmitLatencyTracker {
+public:
+	void note_ingest();
+	void note_emit();
+	void note_emit_stall();
+
+	[[nodiscard]]
+	std::uint64_t emit_stall_events() const;
+
+	[[nodiscard]]
+	std::uint64_t pending_frames() const;
+
+	[[nodiscard]]
+	LatencySummary summarize() const;
+
+private:
+	std::deque<std::uint64_t> ingest_queue_ns_{};
+	std::vector<std::uint64_t> samples_us_{};
+	std::uint64_t emit_stall_events_{0};
+};
+
+}