Docs: require a real-screenshot visual inspection to close out M1

The headless ui_fit_test proves panels *fit* their windows but not that the
overlay *looks* right. Document that closing M1 requires running the actual
coop_host.exe (-DCOOP_TEST_HARNESS=ON), driving it to maximum info, taking an
F10 screenshot, and eyeballing the real overlay -- a green unit test is not a
substitute for looking at the product. Verified live: every panel fits and reads
correctly at the monitor resolution, and the live `uifit` reports "fit".

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

README.md

@@ -139,6 +139,13 @@ Conventions for every milestone below:
   overflow) so later milestones that add UI keep it green. *Independent of the backend work;
   every milestone below must preserve this test* (M2's red banner and Vulkan-layer checkbox
   in particular).
+  - **Mandatory final visual inspection.** The headless `ui_fit_test` proves content *fits*
+    its window, but not that it *looks* right. Before this milestone is considered done, run
+    the **actual `coop_host.exe`** (build with `-DCOOP_TEST_HARNESS=ON`), drive it to its
+    maximum-information state (inject a target, audio on, Debug details on), take an **F10
+    screenshot of the running tool**, and **eyeball the real overlay** — every panel readable,
+    nothing clipped/overlapping, colours/labels correct. A green unit test is not a substitute
+    for looking at the product; this real-screenshot check is required, not optional.
 
 - **M2 — Vulkan (mock → capture).** The largest.
   1. **Mock backend** (`render_vk.cpp`). New submodules **Vulkan-Headers**