Detect locale code that leaves a string literal open

[nank1ro]

A translation pass can drop a \n escape, turning printf("%f\n", x) into a real newline inside the literal, which no longer compiles. Nothing caught this today: the Dart validator never executes type-1 code, check_outputs.py skips type 1, and check_locales.py blanks comment text rather than quotes.

The 12 files this found were already fixed in #312, so it reports zero now — this lands the detector that found them.

How it works

strip_comments in check_locales.py already lexes each language properly (triple-quoted strings, // inside a string, quotes in char literals and comments, backslash escapes), so the checker borrows it rather than matching quotes itself. Two new optional out-params collect what that scan already sees: every line leaving a literal open, and the offset of every /* that never closes.

That second one matters because an unclosed /* is deliberately not comment state — the opener is emitted as text and scanning continues — so an apostrophe in the prose behind it would read as a string opener. en/c/comments/5.md asks the learner to close such a /*, and it/c/comments/5.md writes "il doppio dell'input" behind it. Findings from that offset on are dropped; taking the offset from the lexer keeps a /* inside a string or behind a // from counting.

Seeds holding a [/] placeholder are skipped, since a placeholder can stand for a quote and balance is undecidable; the --solutions-- copy is checked instead. Findings are addressed by (fence, line) so the en baseline excuses only the line en itself trips on, not the whole fence around it.

Verification

• python3 scripts/test_check_locales.py → ok
• python3 scripts/check_string_literals.py → 0 unterminated string literal(s), exit 0

Local-only tool, consistent with the other scripts/check_*.py; not wired into CI.

Reviewed independently (gpt-5.6-sol); both findings it raised — fence-wide suppression hiding locale slips, and non-lexical /* counting truncating valid code — are fixed here with regression tests.