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

CLAUDE.md

@@ -0,0 +1,52 @@
+# card-raytracer — project notes for Claude
+
+This repo is a small study of Andrew Kensler's "business card raytracer".
+The point of the project is **preservation + explanation**, not extension.
+
+## The sacred rule
+
+**`card.cc` must never be modified.** It is a historical artifact, ~1337
+characters of obfuscated C++ that fits on a real business card. Any cleanup,
+formatting, comment, or "fix" goes into `card_explained.cc` instead.
+
+If a compiler warning or modern-C++ issue appears against `card.cc`, the
+answer is always a CMake flag adjustment (e.g. silencing warnings), never
+an edit to the source.
+
+## Layout
+
+| File | Role |
+|---|---|
+| `card.cc` | Original, golfed, untouched. Read-only by convention. |
+| `card_explained.cc` | De-obfuscated rewrite with full names and comments. Same visual output. |
+| `CMakeLists.txt` | Builds both binaries; provides `image` and `image_explained` render targets. |
+| `docs/aek.png` | Published render used by the README. Regenerate after meaningful changes. |
+| `README.md` | Human-facing description with the embedded render. |
+
+## Build & render
+
+```sh
+cmake -S . -B build
+cmake --build build                          # builds both binaries
+cmake --build build --target image           # → build/aek.ppm
+cmake --build build --target image_explained # → build/aek_explained.ppm
+```
+
+Convert PPM → PNG (for README, etc.) with `pnmtopng build/aek.ppm > docs/aek.png`.
+
+## Conventions
+
+- `card_explained.cc` is built with `-Wall -Wextra` and should stay
+  warning-clean. `card.cc` is built with `-w` because we can't edit it.
+- C++14, no external dependencies beyond `libm`.
+- The de-obfuscated version preserves the **same `rand()` call sequence** as
+  the original so the rendered image is visually identical. Don't reorder
+  the `randf()` calls inside `shade()` or the per-sample loop in `main()`
+  without re-checking the output.
+
+## Remote
+
+Hosted on Forgejo: `ssh://git@kode.naiv.no:2222/olemd/card-raytracer.git`.
+Per global instructions, use the `forgejo` skill (invoke via `/forgejo`) and
+the `fj` CLI for any repository interactions (PRs, issues, releases, etc.).
+Never use `gh` for this repo.