/**
 * plainLine — the ONE rule for "one visible line of plain text" that a
 * person will read in a report.
 *
 * Pattern: One predicate, no imports, no state.
 * Role:    Leaf. Asked by the three doors that take a short declared label:
 *          `semantics/coverage/items.ts` (a coverage item's `short`, at the
 *          `absent()` / `coverage()` mint AND at the dispatch reader) and
 *          `injection-engine/factories/defineSkill.ts` (a skill's
 *          `title`). One rule, so the three cannot disagree.
 * Emits:   N/A.
 *
 * ## Why more than CR/LF
 *
 * A report exists to be honest about what a run did, and these labels are
 * printed in it verbatim. "One line" checked as `/[\r\n]/` let through the
 * characters that break a line or a reading WITHOUT being CR or LF:
 *
 *   - other controls (`\p{Cc}`) — VT, FF, NEL (U+0085), TAB;
 *   - line and paragraph separators (`\p{Zl}`, `\p{Zp}`) — U+2028 / U+2029;
 *   - format characters (`\p{Cf}`) — bidi overrides such as U+202E, which
 *     reverse what a reader sees (the "Trojan source" trick), and zero-width
 *     characters, which pad a 5-character label past any length limit while
 *     looking like 5 characters.
 *
 * The rule refuses all four classes, and it runs BEFORE the length is
 * measured, so a length limit is a limit on what a person sees.
 *
 * ## Deliberately strict for v1
 *
 * `\p{Cf}` also refuses marks that legitimate text uses: U+200E / U+200F
 * (LEFT-TO-RIGHT / RIGHT-TO-LEFT MARK, in right-to-left labels) and U+200D
 * (ZERO WIDTH JOINER, in emoji sequences). The report's surfaces are
 * English-only in v1, so nothing honest is lost today. A later locale packet
 * that admits right-to-left labels must allow those three explicitly — and
 * keep refusing the bidi OVERRIDES and isolates (U+202A–U+202E,
 * U+2066–U+2069), which reorder what a reader sees.
 */
/**
 * Why `text` is not one plain visible line, or `undefined` when it is. The
 * sentence names the field (`label`) and the offending code point, so a
 * refusal can be acted on without a hex editor.
 */
export declare function plainLineProblem(label: string, text: string): string | undefined;