Validate single-form plurals against plural source placeholders

[yuefdev]

Related to #661.

A one-form plural translation is currently checked against the singular source. This rejects valid translations when the singular already contains a named placeholder and the plural introduces another:

python_format(
    Catalog('zh'),
    Message(
        ('One file for %(owner)s', '%(num)d files for %(owner)s'),
        ('%(owner)s: %(num)d',),
    ),
)
# TranslationError: unknown named placeholder 'num'

The original no-placeholder example in #661 already passes on master, but this variant still fails. Use the plural source for a plural message in a catalog with exactly one plural form. That translation is used for every count, including plural counts.

Regressions cover Chinese, Japanese and Korean, incompatible/unknown placeholders, and the existing singular validation in a two-form catalog. Four new cases fail before the fix.

Validation: python -m pytest tests/messages/test_checkers.py tests/messages/test_catalog.py -q --tb=short --show-capture=no — 92 passed. Configured pre-commit checks passed for both changed files.

AI assistance: Codex investigated the remaining failure, prepared the fix and tests, and ran these automated checks. No independent human review is claimed.