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.hpp

@@ -0,0 +1,43 @@
+// Debug-only IPC test harness (compiled only when COOP_TEST_HARNESS is defined; the
+// shipped product never contains it). Lets a test script drive the real overlay code
+// paths -- inject, enable audio, re-measure, override, etc. -- by writing a command to
+// %TEMP%\coop_test_cmd.txt and reading the reply from %TEMP%\coop_test_resp.txt, instead
+// of simulating mouse/keyboard input (which ImGui doesn't accept reliably via PostMessage).
+//
+// Protocol: the driver writes one command line to the cmd file; the host consumes it
+// (deleting the cmd file to ack), runs it on the main thread (so it hits the same code
+// the UI buttons do), and writes a single response line to the resp file. One command at
+// a time. See tools/test_drive notes / the per-phase validation scripts.
+#pragma once
+
+#include <string>
+
+namespace coop
+{
+
+class TestHarness
+{
+public:
+#ifdef COOP_TEST_HARNESS
+	void init(); // resolve the %TEMP% file paths and clear any stale command
+	// Main thread: returns the next pending command line (acking by deleting the cmd
+	// file), or an empty string if none is waiting.
+	std::string poll_command();
+	// Main thread: write the response for the command just handled.
+	void write_response(const std::string& resp);
+
+private:
+	std::wstring cmd_path_;
+	std::wstring resp_path_;
+#else
+	// No-op shims so call sites don't need their own #ifdef.
+	void init() {}
+	std::string poll_command()
+	{
+		return {};
+	}
+	void write_response(const std::string&) {}
+#endif
+};
+
+} // namespace coop