Fix #13682: column paste anchors on the first selected row

[niksedk]

Problem

Part 2 of #13682: Column > Paste from clipboard > Replace existing cells works when the rows are selected bottom-to-top, but pastes at the wrong place when the same rows are selected top-to-bottom.

ColumnPasteFromClipboard anchored on Subtitles.IndexOf(SelectedSubtitle). SelectedSubtitle is the moving end of a shift- or drag-selection — SelectGridRange deliberately selects currentIndex first so the edit box and waveform follow the row the user stopped on. So:

• picked bottom-to-top → current row is the top of the selection → anchor correct, paste works;
• picked top-to-bottom → current row is the bottom of the selection → the paste starts there and writes over the lines below the selection instead of over the selection itself.

SE4 used int index = FirstSelectedIndex; (4.0.16:src/ui/Forms/Main.cs:33434), which is direction-independent.

Fix

New MainViewModel.FirstSelectedSubtitleIndex — the topmost selected row, display-only reference rows excluded, -1 when nothing editable is selected. It is SE4's FirstSelectedIndex, built on the already-ordered SubtitleGridSelectedItems.

Both column commands that write downwards from the selection now use it:

• ColumnPasteFromClipboard
• ColumnInsertTextFromSubtitle — same defect, and SE4 anchored it on FirstSelectedIndex too (4.0.16:src/ui/Forms/Main.cs:33726)

The SelectedSubtitle == null guards are gone: the existing idx < 0 check already covers that, and it now also covers a selection consisting only of display-only reference rows, which the old guard let through.

Verification

• New theory in SubtitleGridSelectionOrderTests shift-clicks in both directions through the real headless grid and asserts the anchor is the first selected row while the current row stays the row the user stopped on
• RED check: with the anchor reverted to SelectedSubtitle, the downward case fails and the upward case passes — exactly the asymmetry reported in the issue
• dotnet build src/ui/UI.csproj — 0 warnings, 0 errors
• dotnet test tests/UI/UITests.csproj — 2809 passed, 0 failed

Two earlier full-suite runs each had one failure (WebVttStylePickerViewModelTests.CheckedStylesApplyAsCueClassesOnTheLine on the first), green in isolation and on re-run — the known parallel/shared-settings flake, unrelated to this code.

Scope

This fixes only part 2 of #13682. Part 1 (restoring SE4's direct Ctrl+V overwrite of the selected lines) is untouched and needs its own change in SubtitleGridCopyPasteHelper.Paste — which is also what open PR #13262 edits.

Not addressed here either: the column paste silently truncates when the clipboard has more lines than there are rows below the anchor.

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