docs: reinforce cibuildwheel {package} vs {project} gotcha

Expand gotcha 5 with the case that just cost a second CI cycle: when you
extract an sdist into a subdir and run `cibuildwheel ./<subdir>`, the sdist's
own bundled tests/ are under {package}, not {project}. Document the exit-code-4
"no tests ran" symptom and the local-repro trap (running `cibuildwheel .` from
inside the extracted dir masks the mismatch; reproduce from the parent dir).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: luhenry

CLAUDE.md

@@ -146,13 +146,21 @@ workflow. Don't hand-write the docs YAML unless you need a `comment`/`warning`.
    once on x86. Only the actual bdist needs `ubuntu-24.04-riscv`. Building a codegen
    toolchain (e.g. protoc via Bazel) on riscv is a dead-end; don't attempt it.
 
-5. **cibuildwheel `{project}` vs `{package}` (this cost a full CI cycle).**
+5. **cibuildwheel `{project}` vs `{package}` (this cost a full CI cycle — twice).**
    `{project}` = the dir where cibuildwheel was invoked (copied to `/project` in the
    container). `{package}` = the package path you passed on the CLI (e.g.
-   `cibuildwheel ./protobuf` → `/project/protobuf`). If you stage extra files (a test
-   wheel, a runner script) *inside the package dir you pass*, reference them via
-   **`{package}`**. Symptom of getting it wrong: test step fails with code **127**,
-   `bash: /project/.../run.sh: No such file or directory`.
+   `cibuildwheel ./protobuf` → `/project/protobuf`). When you extract an sdist into a
+   subdir and run `cibuildwheel ./<subdir>`, **everything inside that subdir —
+   including the sdist's own bundled `tests/` — is under `{package}`, not
+   `{project}`**. Reference staged helper files (a test wheel, a runner script) *and*
+   the test suite via **`{package}`** (`pytest {package}/tests`). Symptoms of getting
+   it wrong: test step fails with code **127** `bash: /project/.../run.sh: No such
+   file or directory` (missing script), or code **4** `file or directory not found:
+   /project/tests` + `no tests ran` (pytest pointed at the wrong dir → collects
+   nothing). **Local-repro trap:** if you validate by `cd`-ing into the extracted dir
+   and running `cibuildwheel .`, then `{project}` == the sdist root and
+   `{project}/tests` works — masking the bug. Reproduce the CI shape exactly: invoke
+   `cibuildwheel ./<subdir>` **from the parent dir**, so `{project}` ≠ `{package}`.
 
 6. **Running a real test suite through cibuildwheel:**
    - Stage helper files inside the package dir (cibuildwheel copies that tree into