feat: add README files for various directories to document structure and usage

tests/README.md

@@ -0,0 +1,19 @@
+# Repository Tests
+
+This directory contains tests for deterministic SDK behavior and build/quality
+tooling.
+
+- `c/test_runtime.c` is the host C test program used by `make sw-host-test`.
+- `test_script_tools.py` covers setup, dependency, filelist, warning, metric,
+  archive, and regression-helper behavior.
+
+Add a host C test when changing deterministic runtime or media logic; add a
+Python test when changing scripts or build policy. Run:
+
+```sh
+make sw-host-test
+python3 -m pytest -q
+```
+
+These tests complement, but do not replace, firmware builds, RTL simulation,
+synthesis, timing, or hardware validation.