docs(tdd): fix ten defects the GREEN run found in the language references

The pressure test that #18 and #20 were waiting on. All six original gaps came
back ANSWERED with the lines quoted — and the run then found ten problems in
what I had written. The worst two are mine, not inherited.

**The Python exemplar was the evaluation task.** references/python.md
demonstrated `split_vat` — the exact function the test scenario asks for,
negative-amount case included. The run said so: "as a teaching example that's
over-fitted; as an evaluation it's a leak... this task can't answer it — the
reference handed over the finished artifact." So the run could not measure
whether the guidance transfers, only whether copying works. The exemplar is now
`parse_retry_after`, deliberately unrelated to anything in this estate, and it
states its own contract rather than requiring the reader to divide 12500 by 1.25
to discover it.

**mypy at RED pointed at the stub the same file forbids.** python.md taught that
a pytest ImportError is a legitimate red, then presented `mypy --strict` as a
matched pair without saying that mypy reports the same missing module — so
following both literally, the way to get mypy green at RED is to create the
module. Now stated: that error is the expected red; what must be fixed
immediately is a --strict violation inside the test file you just wrote.

The rest, each quoted in the run:

- The checklist contradicted itself two lines apart — "strike it deliberately"
  above, "Can't check all boxes? You skipped TDD" below. A struck box is by
  definition unchecked, so the Python path routed straight into "start over".
- `match=` was overclaimed as "free". It is `re.search`, so a message containing
  parens quietly matches almost anything; and it couples every error test to
  wording. The Go comparison was a strawman against `assert.Error` when
  `assert.ErrorIs` exists — the honest Python analogue is a custom exception
  type, with `match=` as the fallback.
- `pytest.approx` was listed as the soft-assert analogue. It solves float
  tolerance and has nothing to do with collecting failures; that is
  pytest-subtests or pytest-check, neither of which ships with pytest.
- The whole-shape assertion silently required value equality — on a plain class
  it compares identity and can never pass.
- "When Stuck" is a language-neutral table that prescribed `t.Helper()`.
- `__tracebackhide__` must be a local inside the helper body; at module level it
  does nothing, silently.

Refs #18, #20
Bump-Type: patch

skills/tdd/SKILL.md

@@ -365,7 +365,7 @@ Before marking work complete:
 - [ ] Tests use real code (mocks only if crossing a system boundary)
 - [ ] Edge cases and errors covered
 
-Can't check all boxes? You skipped TDD. Start over.
+Can't check **or deliberately strike** all boxes? You skipped TDD. Start over. A box struck with a stated reason is answered; a box left hanging is not.
 
 ## When Stuck
 
@@ -374,7 +374,7 @@ Can't check all boxes? You skipped TDD. Start over.
 | Don't know how to test | Write wished-for API. Write assertion first. Ask Mathias. |
 | Test too complicated | Design too complicated. Simplify interface. |
 | Must mock everything | Code too coupled. Use dependency injection. |
-| Test setup huge | Extract helpers with `t.Helper()`. Still complex? Simplify design. |
+| Test setup huge | Extract helpers (`t.Helper()` in Go, a local `__tracebackhide__ = True` in Python). Still complex? Simplify design. |
 
 ## Debugging Integration