Log window: stream the hook's logs over IPC into an in-app Log panel

Add a shared log ring (common/coop/log_ring.hpp): a lossy multi-producer /
single-consumer ring named coop_log_<pid>. The hook logs from several threads,
so producers claim a slot with fetch_add and publish each record with a
release store of its sequence; the consumer reads in order and tolerates
losing the oldest lines if it falls a whole ring behind.

The DLL's logf() now formats once and pushes every line to the ring (the file
trace stays as an opt-in mirror); the worker attaches the ring right after IPC
connect so bring-up is captured. The host (IpcServer) creates the ring at
injection time and exposes drain_logs(); a new LogPanel pulls new lines each
frame into a bounded rolling buffer and renders them with auto-scroll, a
filter, and clear. Added to the View menu (and UiState.show_log).

Verified against Phantom Brave via coop_audio_probe, which now also creates the
ring and drains it: the full hook bring-up trace streamed over IPC. All four
tests pass.

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

host/src/log_panel.cpp

@@ -0,0 +1,75 @@
+#include "log_panel.hpp"
+
+#include <cstdio>
+#include <cstring>
+
+#include "imgui.h"
+
+#include "injection_panel.hpp"
+
+namespace coop
+{
+
+void LogPanel::add_line(const LogRecord& rec)
+{
+	if (first_millis_ == 0)
+	{
+		first_millis_ = rec.millis;
+	}
+	const double secs = static_cast<double>(rec.millis - first_millis_) / 1000.0;
+
+	char buf[256];
+	std::snprintf(buf, sizeof(buf), "[%8.3f] %s", secs, rec.text);
+	lines_.emplace_back(buf);
+	while (lines_.size() > kMaxLines)
+	{
+		lines_.pop_front();
+	}
+}
+
+void LogPanel::pull(InjectionPanel& injection)
+{
+	injection.drain_logs([this](const LogRecord& rec) { add_line(rec); });
+}
+
+void LogPanel::draw()
+{
+	ImGui::SetNextWindowPos(ImVec2(24, 760), ImGuiCond_FirstUseEver);
+	ImGui::SetNextWindowSize(ImVec2(720, 240), ImGuiCond_FirstUseEver);
+	ImGui::Begin("Log");
+
+	if (ImGui::Button("Clear"))
+	{
+		lines_.clear();
+		first_millis_ = 0;
+	}
+	ImGui::SameLine();
+	ImGui::Checkbox("Auto-scroll", &autoscroll_);
+	ImGui::SameLine();
+	ImGui::SetNextItemWidth(-1.0f);
+	ImGui::InputTextWithHint("##logfilter", "filter...", filter_, sizeof(filter_));
+
+	ImGui::Separator();
+	if (ImGui::BeginChild("loglines", ImVec2(0, 0), ImGuiChildFlags_None, ImGuiWindowFlags_HorizontalScrollbar))
+	{
+		const bool has_filter = filter_[0] != '\0';
+		for (const std::string& line : lines_)
+		{
+			if (has_filter && line.find(filter_) == std::string::npos)
+			{
+				continue;
+			}
+			ImGui::TextUnformatted(line.c_str());
+		}
+		// Stick to the bottom while new lines arrive (unless the user scrolled up).
+		if (autoscroll_ && ImGui::GetScrollY() >= ImGui::GetScrollMaxY() - 1.0f)
+		{
+			ImGui::SetScrollHereY(1.0f);
+		}
+	}
+	ImGui::EndChild();
+
+	ImGui::End();
+}
+
+} // namespace coop