Audio: operator re-measure + format override (host<->hook op channel)

Add a per-stream op channel in AudioRingHeader (op_seq + op_* fields, version
2): the host posts re-measure / override commands, the hook applies them and
re-publishes (bumping format_generation), and the host rebuilds its render
client live on the change. The Audio panel (under Debug details) gains a
"Re-measure rate" button and a rate/channels/bit-depth/format override -- for
when detection is wrong or the channels/bit-depth were unrecoverable.

Also add a debug-only IPC test harness (-DCOOP_TEST_HARNESS, off by default,
absent from the shipped host): a file-based command channel that drives the
host's real UI code paths (inject / audio / re-measure / override / screenshot
/ status) for scripted validation, instead of unreliable synthetic mouse input.
Used to validate live: late-attach to coop_tone@44100 -> measured 44100,
promoted to hooked; override -> 2ch state, re-measure -> reconverge.

Trim the README roadmap to what's left; document the harness + rate_estimator_test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/test_harness.cpp

@@ -0,0 +1,51 @@
+#include "test_harness.hpp"
+
+#ifdef COOP_TEST_HARNESS
+
+#include <fstream>
+
+#include <windows.h>
+
+namespace coop
+{
+namespace
+{
+std::wstring temp_file(const wchar_t* name)
+{
+	wchar_t dir[MAX_PATH] = {};
+	const DWORD n = GetTempPathW(MAX_PATH, dir);
+	return (n != 0 && n < MAX_PATH) ? std::wstring(dir) + name : std::wstring(name);
+}
+} // namespace
+
+void TestHarness::init()
+{
+	cmd_path_ = temp_file(L"coop_test_cmd.txt");
+	resp_path_ = temp_file(L"coop_test_resp.txt");
+	DeleteFileW(cmd_path_.c_str()); // drop any stale command from a previous run
+	DeleteFileW(resp_path_.c_str());
+}
+
+std::string TestHarness::poll_command()
+{
+	std::ifstream f(cmd_path_.c_str()); // MSVC accepts a wide path
+	if (!f)
+	{
+		return {};
+	}
+	std::string line;
+	std::getline(f, line);
+	f.close();
+	DeleteFileW(cmd_path_.c_str()); // ack: the command has been taken
+	return line;
+}
+
+void TestHarness::write_response(const std::string& resp)
+{
+	std::ofstream f(resp_path_.c_str(), std::ios::trunc);
+	f << resp;
+}
+
+} // namespace coop
+
+#endif // COOP_TEST_HARNESS