Add d3d9_hook_test (capture path + D3D9 present-overhead guard)

The D3D9 capture path had no dedicated test. d3d9_hook_test plays game + host:
installs the IDirect3DDevice9::Present hook, drives a real D3D9 device (Clear +
Present), and verifies the full GetRenderTargetData -> BGRA->RGBA swizzle ->
shared keyed-mutex texture path by opening coop_video_<pid> from a D3D11 device
and reading the rendered color back ({51,102,153}).

It also carries the present-thread overhead guard the perf section deferred here,
at a realistic 1280x720: measured capture overhead is ~0.39 ms -- far under one
frame, confirming the cached-memory D3D9 read-back is not a stall (and tripping
the budget if a write-combined-class regression ever lands on the present thread).
Skips cleanly without a D3D9/D3D11 device.

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

README.md

@@ -114,7 +114,7 @@ From an in-depth review pass. Each item is fixed test-first (a failing test, the
 as its own commit; "verify" items are confirmed real before any change, and dropped if not.
 
 Test coverage:
-- **Dedicated hook tests** — `focus_spoof`, `vk_hook` (present), `d3d9_hook`.
+- **Dedicated hook tests** — `focus_spoof`, `vk_hook` (present). (`d3d9_hook` done.)
 
 Features:
 - **Static CRT (`/MT`) for `coop_hook.dll`** (x64 + x86) so it loads in games without the VC++ redist.