Clear the fuzzy flag when we write a real translation

[arvida]

• msgfmt excludes fuzzy entries from the compiled catalog, so a stale #, fuzzy on an entry we translated made the app fall back to the source string
• Existing-file write-back: record the entry being translated by its own key when each change is registered, then rewrite its #, lines, walking backwards so a removed line cannot shift an unvisited one
• New-file creation: strip fuzzy once after all plural forms are in, so the outcome does not depend on their order
• Other flags on the same line and the #| previous-msgid block are left as found
• Regression tests for each path, each shape of plural key, several flag lines per entry, and the plural-form ordering cases