Initial commit: business card raytracer with explanation

Preserves Andrew Kensler's original card.cc verbatim and adds:
- CMakeLists.txt building both the original and a de-obfuscated
  variant (card_explained.cc) that produces a visually identical render
- A heavily annotated rewrite explaining the vector ops, ray-sphere
  intersection, soft shadows, depth of field, and reflection recursion
- Rendered sample output (docs/aek.png) embedded in the README
- CLAUDE.md establishing the "never modify card.cc" rule for future work

CMakeLists.txt

@@ -0,0 +1,43 @@
+cmake_minimum_required(VERSION 3.16)
+project(card_raytracer CXX)
+
+# The original card.cc is sacred and must NOT be modified. We compile it
+# verbatim. Because the file is golfed C++ from the early 2010s, we relax
+# a few things so a modern compiler accepts it untouched:
+#   - C++14 (default constructor v(){} is fine; pow(float,int) overload exists)
+#   - -w  silences the unavoidable warnings (implicit float->int conversions
+#         in the final printf, narrowing, unused result of operator new, etc.)
+#         We can't fix them — we'd have to edit card.cc, which we won't.
+set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+add_executable(card card.cc)
+add_executable(card_explained card_explained.cc)
+
+# Per-compiler flags. The original is golfed and triggers warnings we can't
+# fix without editing it (forbidden). The de-obfuscated version keeps full
+# warnings on so it stays honest.
+if(MSVC)
+    target_compile_options(card           PRIVATE /w /O2)
+    target_compile_options(card_explained PRIVATE /W4 /O2)
+else()
+    target_compile_options(card           PRIVATE -w -O3)
+    target_compile_options(card_explained PRIVATE -Wall -Wextra -O3)
+endif()
+
+# Convenience targets: render an image from each binary.
+# Run with:  cmake --build build --target image
+add_custom_target(image
+    COMMAND card > aek.ppm
+    DEPENDS card
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+    COMMENT "Rendering aek.ppm from the original card (512x512 @ 64 spp)"
+)
+
+add_custom_target(image_explained
+    COMMAND card_explained > aek_explained.ppm
+    DEPENDS card_explained
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+    COMMENT "Rendering aek_explained.ppm from the de-obfuscated card"
+)