Remove the literal % from [tool.check_test_layout].reason — it crashes configparser-based tools

[tschm]

Subcategory: tooling robustness / cross-tool compatibility

The reason string in [tool.check_test_layout] contains a literal 100%. Tools that read
pyproject.toml's [tool.*] tables through Python's configparser treat % as interpolation
syntax and crash outright — not degrade, crash — on any invocation inside this repo.

Evidence — uvx radon cc src -a -s from the repo root:

File ".../radon/cli/__init__.py", line 85, in file_config
    config.read_dict(FileConfig.toml_config())
  File ".../configparser.py", line 440, in before_set
    raise ValueError("invalid interpolation syntax in %r at position %d" ...)
ValueError: invalid interpolation syntax in 'Tests are organised by behaviour under
tests/test_jquantstats/; per-module coverage is guaranteed by the 100% pytest coverage gate
rather than 1:1 file mirroring.' at position 108

Both radon cc and radon mi fail this way. The complexity metrics for this repo's
/rhiza:quality assessment had to be gathered by running radon from a different working
directory with an absolute path — the numbers themselves were fine (average CC A/2.83, no block
ranked C or worse, every module A on maintainability index), but nobody running radon normally
would get that far.

Change — pyproject.toml, [tool.check_test_layout].reason: replace 100% with wording
that avoids the character, e.g. "…guaranteed by the full-coverage pytest gate…" or "…by the
pytest coverage gate at its configured threshold…". Escaping as 100%% also satisfies
configparser but reads wrong to anyone else and would show up literally in other tools.

The opt-out itself is correct and should stay — check_test_layout.py accepts it and exits 0
with "parity not enforced by request".

Done when: uvx radon cc src -a -s runs to completion from the repo root.

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Filed from a /rhiza:quality run on 2026-08-01.