v0.3.1 — xlsx: valid frozen panes, unique sheet names

Fixes Excel's "we found a problem… repair?" prompt on exported workbooks.

• Frozen panes: activePane must name a pane the split actually creates. A header-row-only freeze (any grid without pinned-left columns) needs bottomLeft, column-only needs topRight; we always claimed bottomRight, and Excel repaired the View record.
• Sheet names: multi-sheet workbooks could emit duplicate names (Excel treats that as corruption). Now deduplicated within the 31-char cap; illegal edge apostrophes stripped.

Both forms are accepted by lenient readers, which is why v0.3.0 passed validation — the pane rules are now asserted against Excel's own semantics for all three freeze shapes.

npm add https://github.com/methodify/augrid/releases/download/v0.3.1/augrid-core-0.3.1.tgz \
        https://github.com/methodify/augrid/releases/download/v0.3.1/augrid-react-0.3.1.tgz