Add StreamingXlsxWriter, a constant-memory xlsxwriter wrapper that writes one
sheet per chart row-by-row, so multi-chart dashboard exports never hold more
than one row per sheet in memory. Includes sheet-name sanitization (forbidden
chars, 31-char cap, reserved 'History', case-insensitive de-duplication) and
per-cell sanitization (formula-injection quoting mirroring excel.quote_formulas,
Excel int/float precision limits, ISO temporal rendering). Reuses the neutral
document properties from utils.excel. Adds unit tests with an openpyxl
round-trip.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
