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/common.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <cstddef>
+#include <string_view>
+
+namespace cvmmap_streamer {
+
+void print_help(std::string_view executable);
+bool has_help_flag(int argc, char **argv);
+std::size_t sample_ipc_payload_size();
+void pipeline_tick();
+void protocol_step();
+
+}