[widechaos]

Fixes #496.

Changes proposed in this pull request:

• Preserve decimal-string digits with Decimal when ndigits is omitted and the string has no exponent, then apply the existing grouping and locale separators. For example, "1234567890123456.78" becomes "1,234,567,890,123,456.78".
• Document that this also retains trailing decimal zeros and uses fixed-point output for tiny plain decimal strings. Explicit ndigits, exponent inputs, invalid-string fallback and non-finite handling keep their existing paths.
• Add precision regressions, unchanged-behavior controls, and German/French locale coverage. Translation binaries were generated locally.

Validation: the precision tests fail on the original code; the patch passes all 893 tests, 15 separate doctests, Ruff, Black and mypy. An independent check with a different long fractional string also passed in English, German and French.

The unmodified tox -e py command hit 15 benchmark setup errors with pytest 9.2.0 / pytest-codspeed 5.0.4 (Function._nodeid was removed). Recreating that single tox environment with a local pytest 9.0.2 installation constraint passed all 893 tests against the installed sdist. No repository dependency or tox configuration changes are included. The complete source-environment suite also passed with network access disabled. Windows, the multi-interpreter tox matrix, the full prek hook set and docs build were not run.