refactor: keep Claro in one strict C89 source file

CONTRIBUTING.md

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+# Contributing to Claro
+
+Claro is a learning tool first. Read [`docs/LEARNING_FIRST.md`](docs/LEARNING_FIRST.md)
+before proposing learner-facing changes. Keep each change small and predictable,
+teach one recommended syntax per concept, preserve supported compatibility
+syntax, and prioritize runnable examples and plain-language diagnostics over
+feature breadth. Do not claim accessibility validation without testing with
+intended learners.
+
+For behavior changes, add a focused regression before implementation and verify
+that it fails for the intended reason. Then build and run the affected examples
+and the repository validation gates. See [`docs/ROADMAP.md`](docs/ROADMAP.md) for
+how the current charter relates to older planning documents.
+
+## Repository layout
+
+- `src/claro.c` — C89 interpreter.
+- `lessons/` — ordered beginner lessons.
+- `examples/` — programs to try and change.
+- `docs/` — learning guides, reference, and historical notes.
+- `tests/` — regression fixtures and expected output.
+- `tools/` — validation and release helpers.
+- `lib/` — existing library helpers.
+- `support/` — optional editor support.
+
+## Build and verify
+
+At the beginning of a development/release run, stamp its UTC version once:
+
+```bash
+python3 tools/stamp_version.py
+make -s all
+```
+
+Ordinary rebuilds use `make -s all` without changing the run timestamp.
+Validation helpers also require Python 3. Run:
+
+```bash
+./claro test
+./claro doctor
+./claro validate
+python3 tools/validate_v1_17.py
+```
+
+The last command is the current full validation entry point; its filename is
+historical. Use focused sanitizer checks for relevant C runtime changes and
+report known failures explicitly.
+
+Execute teaching examples with suitable input and assert their expected output;
+syntax checking alone is not enough. Update the relevant guide, rerun checks
+after the final edit, and finish with `git diff --check`. Preserve unrelated
+local work; exclude generated binaries, caches, and unverified experiments
+from commits. See [versioning](docs/VERSIONING.md) for the timestamp convention.
+
+
+The runtime must remain a single C89 implementation file. The full gate runs
+`python3 tools/validate_c89.py` to compile it independently with strict C89 flags
+and test formatting, numeric boundaries, and simple functions. Do not introduce
+C99 syntax or depend on generated project headers. See [C89 notes](docs/C89.md).