[HyakoV3]

Part of #1447 (items 1, 3, 4, 5 and 6 of its list; the issue stays open for 2 and 7).

All in crates/xlsx/src/pivot.rs:

1. Unopenable file: booleans and errors in the source count as strings in the cache <sharedItems> flags, as Excel writes them. Before, containsString="0" next to them made Excel refuse the file (even with repair).
2. Report filter: pageField@item is read through the pivot field's <items> (x = shared item), so Qty = 3 is no longer read as another item.
3. Date groups with a blank date: containsSemiMixedTypes="0" is only written when there is no blank.
4. Day groups: a date-grouped field is written without sortType (Excel sorted its labels as text) and a date group without sortType reads back as ascending.
5. Two value fields, no column field: firstHeaderRow="0" plus <x14:pivotTableDefinition hideValuesRow="1"/>, so Excel 365 adds no Values row (no more #SPILL! below the report). One value field is unchanged.

Not in this PR: 2 (source headers with leading/trailing spaces: engine + xlsx header handling) and 7 (PivotChart link needs the file name threaded to the writer). Happy to do them next; they touch other files.

Tests: cache flags for the six column kinds of the issue's table, date-group flags with and without a blank, hideValuesRow, and the report-filter read. The three write-side tests fail on main. The existing round-trip fixture's Years group now uses PivotSort::Asc (the default) since a date group without sortType reads as ascending. cargo xtask ci passes.

Open PRs #722, #474, #498, #719 and #684 touch the same files, so a rebase may be needed.

🤖 Generated with Claude Code

[HyakoV3]

Split into one PR per item of #1447, as the ticket asked: #1680 (item 3), #1681 (item 4), #1682 (item 5), #1683 (item 6). Item 1 (Boolean/error cache flags) is left out here.