[jstet]

Closes #239.

columns-n (and plain columns) on select_one / select_multiple now becomes LimeSurvey's display_columns question attribute on L and M questions, so a long option list is laid out in n columns. The way back reads display_columns=n as columns-n.

Design:

• The count is part of the appearance name, so this is a pattern, not a registry allowlist entry. I kept it out of the registry (no schema or codegen change, no generic "attribute from appearance" mechanism for a single user) and put it in src/conventions/displayColumns.ts, which the appearance handler, the validator and the reverse path share. display_columns is added to ATTRIBUTE_COLUMNS.
• Valid n is 1 to 8. LimeSurvey's attribute definition (questionHelper.php, inputtype columns, default 1) states no maximum; 8 is the largest count its column selector offers and the renderer just adds the class nbcol-n. Anything else (columns-0, columns-9) warns with the new code appearance-invalid-columns and writes nothing. columns-x stays appearance-unregistered.
• Plain columns means "as many as fit" in ODK, which LimeSurvey cannot do, so it becomes 2 (and reads back as columns-2).
• Precedence: an explicit column count wins over horizontal (the button theme is dropped, as a dropdown appearance already wins over it). A dropdown (minimal, autocomplete) has no columns: none is written. On other types columns-n warns appearance-invalid-for-type.
• Not checked: how fruity_twentythree renders nbcol-n at phone width (no LimeSurvey instance used).

Before / After:

• Before: columns-2 warned appearance-unregistered and was ignored.
• After: columns-2 on a select gives display_columns=2; the TSV round-trips to columns-2.

Tests: new tests/ts/unit/displayColumns.test.ts (17: mapping for one/multiple/plain/n=8, no column when unused, invalid n, wrong type, horizontal precedence, dropdown, round trips) and a appearance-invalid-columns case in diagnosticCodes.test.ts. npm test: 1791 passed (109 files); npm run validate passes.

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