HF-85: Implement DCOUNT database function

[marcin-kordas-hoc]

Summary

• New DatabasePlugin with DCOUNT(database, field, criteria) implementation
• Counts numeric values in a field of a database range that match specified criteria
• First database function — shared helpers ready for DSUM, DAVERAGE, DMAX, DMIN, etc.

Changes

• New: src/interpreter/plugin/DatabasePlugin.ts (208 lines)
• Modified: src/interpreter/plugin/index.ts (export added)
• Modified: 16 language files in src/i18n/languages/ (DCOUNT translations)
• Modified: CHANGELOG.md (unreleased entry)

Implementation Details

• 3 reusable private helpers: resolveFieldIndex(), buildDatabaseCriteria(), rowMatchesCriteria()
• Field resolution: string (case-insensitive header match) or 1-based numeric index with Math.trunc()
• Criteria: OR across rows, AND within each row, using CriterionBuilder.fromCellValue()
• Counts only isExtendedNumber() values
• Returns 0 when no records match (not error)

Excel Validation

• 10/10 PASS in validation workbook (Excel Desktop, PL locale)
• Verified: field index, boolean criteria, multiple AND criteria, error conditions

Tests

• 16 test cases in hyperformula-tests (matching branch)
• Groups: core sanity, edge cases, boolean coercion, multiple criteria, errors, float truncation

Review Notes

• Reviewed by Claude (Sonnet) + Codex (gpt-5.4) in parallel
• 2 P1 bugs found by Codex and fixed: RichNumber criteria normalization, empty string handling
• CHANGELOG entry included
• Float field index truncation (Math.trunc) added per review

Linked

• Tests PR: handsontable/hyperformula-tests (manual — private repo)
• ClickUp: HF-85

[qunabu]

Task linked: HF-85 Implement function DCOUNT

[github-actions]

Performance comparison of head (2e2ecec) vs base (357eee3)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 479.87 | 484.72 | +1.01%
                                      Sheet B | 152.79 | 155.37 | +1.69%
                                      Sheet T |  136.8 |  136.9 | +0.07%
                                Column ranges | 466.33 | 467.25 | +0.20%
Sheet A:  change value, add/remove row/column |  14.57 |  14.63 | +0.41%
 Sheet B: change value, add/remove row/column |  123.3 | 128.29 | +4.05%
                   Column ranges - add column |  142.4 | 142.94 | +0.38%
                Column ranges - without batch | 430.06 |  428.2 | -0.43%
                        Column ranges - batch | 110.12 |  110.2 | +0.07%