fix: keep prior selection when shift-clicking after ctrl-click

[spike-rabbit]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Closes #582. Shift-clicking after a Ctrl-click discards previously selected rows because the range selection replaces the selection with [].

What is the new behavior?

Shift-click extends the existing selection with the range from the last clicked row, preserving prior Ctrl-selections. Duplicates are skipped.

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

Other information:

Added a unit test in body.component.spec.ts covering the issue's repro steps.

[gemini-code-assist]

Code Review

This pull request fixes a regression where shift-clicking to select a range would clear previously selected rows. The changes ensure that the current selection is preserved when calculating the new range and add a guard for the previous selection index. A performance optimization was suggested for the selection utility to use a Set for membership checks instead of Array.includes() to avoid quadratic time complexity during large selections.

[Copilot]

Pull request overview

Fixes multi-row selection behavior so that shift-click range selection preserves any prior ctrl/meta selections (issue #582), and adds coverage for the regression.

Changes:

• Update shift-click selection to extend the current selection instead of replacing it with an empty selection.
• Add de-duplication when adding a shift-click range to the selection.
• Add unit tests reproducing ctrl-click + shift-click behavior and a “first click is shift” edge case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
projects/ngx-datatable/src/lib/utils/selection.ts	Adds de-dupe logic when selecting ranges.
projects/ngx-datatable/src/lib/components/body/body.component.ts	Preserves existing selection when shift-selecting, guarded by prevIndex.
projects/ngx-datatable/src/lib/components/body/body.component.spec.ts	Adds regression tests for ctrl+shift multi-selection behavior.

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