Scrub 'OLD code' comment framing and a getenv warning from tooling

The vk_capture_perf_test comments described its synchronous reference
arm as "the OLD code"; it is a deliberately-constructed slow baseline
the test compares against, not removed history -- reworded to say so.

Replace the last std::getenv (tone_source.hpp) with
GetEnvironmentVariableA, dropping the C4996 deprecation warning from
every audio-tone build, and drop the now-unused cstdint/cstdlib
includes. Generalize a game-name aside in the header comment.

tests/vk_capture_perf_test.cpp

@@ -8,13 +8,13 @@
 //
 // This test builds a known gradient image (in PRESENT_SRC layout) on a real device and measures the
 // time the *present thread* spends per capture for two implementations:
-//   * a synchronous reference that mirrors the OLD code (copy + WaitForFences + read the
+//   * a synchronous reference (copy + WaitForFences + read the HOST_COHERENT mapping + swizzle, all
-//     HOST_COHERENT mapping + swizzle, all inline) -> reproduces the stall, and
+//     inline) -> reproduces the stall, and
-//   * coop::hook::VkCapture (the fix: present thread only records+submits; a reaper thread does the
+//   * coop::hook::VkCapture (present thread only records+submits; a reaper thread does the
 //     HOST_CACHED read-back + swizzle + upload off the critical path).
-// It asserts the fixed present-thread cost is a small fraction of the synchronous cost, and that the
+// It asserts VkCapture's present-thread cost is a small fraction of the synchronous cost, and that
-// captured image is byte-correct (BGRA->RGBA swizzle). `--sync` routes the measured path through the
+// the captured image is byte-correct (BGRA->RGBA swizzle). `--sync` routes the measured path through
-// synchronous reference so the same assertion FAILS, demonstrating the test catches the regression.
+// the synchronous reference so the same assertion FAILS, demonstrating the test catches the regression.
 //
 // Needs a working Vulkan ICD (the dev box has one). With no vulkan-1.dll / no device it SKIPs.
 #include <cstdint>
@@ -391,7 +391,7 @@ int main(int argc, char** argv)
 		submit_wait(cb);
 	}
 
-	// --- Synchronous reference (mirrors the OLD code: HOST_COHERENT staging, inline read-back) ------
+	// --- Synchronous reference (HOST_COHERENT staging, inline read-back on the present thread) ------
 	VkBuffer ref_buf = VK_NULL_HANDLE;
 	VkDeviceMemory ref_mem = VK_NULL_HANDLE;
 	void* ref_mapped = nullptr;
@@ -443,8 +443,8 @@ int main(int argc, char** argv)
 		}
 	};
 
-	// Time the synchronous reference (a few iterations; this is the per-present cost the OLD code put
+	// Time the synchronous reference (a few iterations; this is the per-present cost an inline
-	// on the game's present thread).
+	// read-back puts on the game's present thread).
 	sync_capture(); // warm
 	double sync_ms = 0;
 	const int iters = 8;
@@ -527,7 +527,7 @@ int main(int argc, char** argv)
 	}
 
 	// The core assertion: the measured present-thread cost must be a small fraction of the synchronous
-	// read-back cost (the fix moves the read-back off the present thread). In --sync repro mode the
+	// read-back cost (VkCapture moves the read-back off the present thread). In --sync repro mode the
 	// measured path IS the synchronous one, so this fails -- demonstrating the test catches the bug.
 	std::printf("present-thread cost: measured %.3f ms vs synchronous %.2f ms (ratio %.3f)\n", sut_ms, sync_ms,
 	            sut_ms / sync_ms);

tools/audio_tone/tone_source.hpp

@@ -1,13 +1,11 @@
 // Configurable WASAPI sine-tone render source, shared by coop_tone.exe and the audio
 // render-hook self-test. Opens a shared-mode render client at a requested format
 // (sample rate / channels / bits / float vs PCM) using AUDCLNT_STREAMFLAGS_AUTOCONVERTPCM,
-// so it can render formats that differ from the device mix format -- exactly how games
+// so it can render formats that differ from the device mix format -- exactly the case a
-// like Godot render 44100 Hz on a 48000 Hz endpoint, the case the hook must detect.
+// game rendering 44100 Hz on a 48000 Hz endpoint creates, which the hook must detect.
 #pragma once
 
 #include <cmath>
-#include <cstdint>
-#include <cstdlib>
 
 #include <windows.h>
 
@@ -91,7 +89,8 @@ public:
 		// Optional: give each channel genuinely different content (a per-channel frequency scale),
 		// so a downstream test can *recover* the channel count by correlation (identical channels
 		// are ambiguous: 2ch@R looks like 1ch@2R). Off by default -> the usual single-tone source.
-		if (const char* d = std::getenv("COOP_TONE_DISTINCT_CH"); d != nullptr && d[0] == '1')
+		char d[2] = {};
+		if (GetEnvironmentVariableA("COOP_TONE_DISTINCT_CH", d, sizeof(d)) > 0 && d[0] == '1')
 		{
 			distinct_ = true;
 		}