fix: remove indexFieldMap entry by field key, not running counter

[Aias00]

Summary

Fixes #976.

When a WriteHolder excludes a column, the ignored branch removed from indexFieldMap using the running loop counter index instead of the field's own sortedFieldMap key. For explicit-index fields, that key (entry.getKey()) equals the field's @ExcelProperty(index) — its indexFieldMap key — so removing by the counter dropped an unrelated explicit-index entry and left the ignored field's entry in place.

Root cause

ClassUtils.doDeclaredFields:

if (writeHolder.ignore(field.getFieldName(), entry.getKey())) {
    ignoreSet.add(field.getFieldName());
    indexFieldMap.remove(index);   // BUG: 'index' is the running counter, not the field's key
} else {
    ...
}

indexFieldMap is keyed by the field's explicit @ExcelProperty(index = N) (see declaredOneField), and for explicit-index fields the sortedFieldMap position entry.getKey() equals that index (see buildSortedAllFieldMap). So the ignored field's own entry is keyed by entry.getKey(), not the counter.

Fix

indexFieldMap.remove(key);

For a field with an explicit index, key == entry.getKey() is its indexFieldMap key, so its own entry is removed; for a field without an explicit index, key is not in indexFieldMap, so the call is a no-op (correct — it was never there).

The corruption is observable downstream: ExcelHeadProperty.initColumnProperties passes indexFieldMap.containsKey(entry.getKey()) as forceIndex into each Head, which DefaultAnalysisEventProcessor reads to drive head-to-column matching.

Verification

Added ClassUtilsTest.test_declaredFields_WriteHolder_exclude_preservesUnrelatedExplicitIndex: with ComplexEntity (id index 0, name index 2, email order 10), excluding email must keep indexFieldMap as {0: id, 2: name}. Before the fix id (index 0) was wrongly removed; after the fix it stays. Full fesod-sheet suite: Tests run: 668, Failures: 0.

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