Drop two placeholder false positives in check and fail on a source with no files

[arvida]

Part of localheroai/localhero-ai#829 (found in a 40-repo run of check over popular open-source apps).

• gettext plurals: a plural form may use a placeholder only the plural source has. wagtail's msgid "Moved down 1 place." has no placeholder; msgid_plural "Moved down %(counter)s places." does; an Arabic first form using %(counter)s was flagged. The allowance is built once per catalog from all source forms of the entry. A placeholder missing from a form is still reported.
• ICU: {name} placeholders compare by presence, since ICU's # stands in for the plural argument. Rails %{}, i18next {{}}, Python and printf still compare counts, so a dropped repeat (a second %{days_in_month}) is still caught.
• --source with no files for that locale now fails with a message instead of "0 keys, 100% complete". An unparseable source file still goes through the parse-failure path (reported, exit 1, --json still printed).

Verified

• Tests first; full suite passes, lint and build clean.
• Real repos: wagtail 71 → 0 flags, twenty 3 → 0, documenso 9 → 7, discourse 209 → 205 (the remaining ones are real, e.g. %{count} renamed to %{number}).
• Regression over 20 local repos with and without config: identical except the monorepo change already on main.
• Review (code-reviewer agent) found one blocker, an unparseable source file being reported as missing with empty --json output; fixed with a test.

Not in this PR

• vue-i18n a | b plural strings are still compared as one string.
• Non-ASCII placeholder names like {aquí} are not recognised.
• The ICU presence rule applies to every single-brace placeholder, not only plural arguments, so {d} of {d} → {d} passes. Rare outside plurals; could be narrowed to names inside a reduced plural/select.