f59a592 left the exact defect class this repo keeps finding in documents: the
docstring and the run said 50 cells while `--selftest`'s argparse help still
said 42, and nothing failed. Two changes:
- the help string no longer carries a count ("the run prints its cell count"),
so it cannot drift again;
- DOC_CELLS parses the docstring's `fixture corpus, N cells` claim and cell 27
grades it against the count that actually ran. Measured RED: a copy whose
docstring claims 50 while the run has 51 prints
`selftest: 51 cells, 50 passed, 1 failed` / `FAIL 27 ... got: 50 want: 51`,
rc 1.
Selftest 50 -> 51 cells, 51/51, rc 0.