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

README.md

@@ -0,0 +1,88 @@
+# card-raytracer
+
+A study of [Andrew Kensler's "business card raytracer"][aek] — a complete
+recursive path-tracer with soft shadows, depth of field, anti-aliasing,
+reflections, a checker floor, and a sky gradient, all crammed into ~1337
+characters of obfuscated C++ small enough to print on the back of a
+business card.
+
+The original code is preserved verbatim as [`card.cc`](card.cc). A
+de-obfuscated, heavily-annotated rewrite lives in
+[`card_explained.cc`](card_explained.cc). Both produce the same image:
+
+![Rendered output: the letters "aek" as reflective spheres on a red and white checker floor under a blue-purple sky, with soft shadows and depth-of-field blur](docs/aek.png)
+
+The spheres spell **a · e · k**, Andrew Kensler's initials, encoded as a
+9-column bitmap in the `G[]` array of the original source.
+
+## Quick start
+
+Requires a C++14 compiler, CMake ≥ 3.16, and (optionally) `netpbm` for
+PPM → PNG conversion.
+
+```sh
+cmake -S . -B build
+cmake --build build
+
+# Render with the original (PPM, 786 KiB, ~6 s on a modern laptop)
+./build/card > aek.ppm
+
+# Or via the convenience target:
+cmake --build build --target image            # → build/aek.ppm
+cmake --build build --target image_explained  # → build/aek_explained.ppm
+
+# View it (PPM is supported by most image viewers; or convert):
+pnmtopng aek.ppm > aek.png
+```
+
+## What's interesting about it
+
+The whole renderer is built on **operator overloading**: a single `struct v`
+provides `+` (add), `*` (scale), `%` (dot product), `^` (cross product), and
+unary `!` (normalise). Once those are in place, the entire rendering
+pipeline — sphere intersections, reflection vectors, Phong shading,
+checker-pattern tile lookup, camera basis construction — collapses into
+just a handful of expressions.
+
+Other tricks worth admiring:
+
+- **Soft shadows for free**: the light position is jittered each call, so
+  averaging 64 samples per pixel produces a real penumbra without any
+  area-light formalism.
+- **Depth of field for free**: the camera origin is also jittered across
+  a 99-unit "lens" each sample, so points off the focal plane go blurry.
+- **Anti-aliasing for free**: sub-pixel jitter on the ray direction.
+
+The same 64-sample loop pays for all three at once.
+
+A line-by-line walkthrough of the algorithm is the long-form
+[comments inside `card_explained.cc`](card_explained.cc).
+
+## Layout
+
+```
+.
+├── card.cc            # original, untouched — do not edit
+├── card_explained.cc  # de-obfuscated, documented rewrite
+├── CMakeLists.txt
+├── docs/
+│   └── aek.png        # published render
+├── README.md
+└── CLAUDE.md          # working notes for AI assistance
+```
+
+## Credits
+
+- Original raytracer: [Andrew Kensler][aek-home] ("aek").
+- Excellent walkthrough that inspired this study:
+  Fabien Sanglard, [_Decyphering The Business Card Raytracer_][aek].
+
+## Licence
+
+The original `card.cc` is reproduced as-is from publicly published material
+for educational purposes; credit and copyright belong to Andrew Kensler.
+The supporting files in this repository (CMake, README, de-obfuscated
+rewrite) are placed in the public domain by the repository author.
+
+[aek]: https://fabiensanglard.net/rayTracing_back_of_business_card/
+[aek-home]: https://www.cs.utah.edu/~aek/