git_ui: Add multi-select to git panel for bulk stage, unstage, and discard

[justinschmitz97]

Closes #26430
Complements: #52935 by Jacob Binnie

Enables multi-select in the Git Panel, following the Project Panel rules.

diff-bulk.mp4

Core selection gestures:

1. Click one file, shift+click a file deeper in the list: all files between highlight.
2. Click one file, then ctrl/cmd+click another file: it joins the selection; or removes it when selected.
3. Shift+down extends the selection from the current row; shift+up shrinks it back and re-extends past the anchor.
4. Escape with a selection clears it and keeps focus; Escape again toggles focus as before.
5. Plain click collapses selection to file and opens its diff; double-click triggers the alternate open action.
6. Shift+click on a checkbox still performs the pre-existing range staging.

Bulk operation gestures:

1. Select multiple files, press space: if any member is unstaged, all become staged; press space again: all unstaged.
2. Right-click a file inside the selection: menu reads Stage/Discard/Trash N files, depending on context.
3. Select multiple files, press delete: confirmation prompt. Mixed selections: untracked = trashed, staged = unstaged.

Folders, in tree view:

1. Ctrl/cmd+click a folder row behaves like its checkbox does: selects it as a unit, expands operations to descendants.
2. Folder rows inside a shift range join the selection. A collapsed folder is selected including its hidden files.
3. The same directory under Tracked and Untracked is two independent selections (so marks are section-qualified).
4. Plain click on a folder still expands/collapses it and clears the selection.

Implementation notes:

1. Selection is stored as repo paths plus section-qualified tree keys to survive status refreshes.*
2. Stale entries are pruned on rebuild, selection is cleared on active repository change.
3. Range selection follows visual order in both flat and tree view.
4. Bulk operations batch into a single git operation.

* Not sure if this is the best option for storing it.

Why this is necessary

• Multi-select has been an ongoing feature wish since 2025 with various upvotes
• Current state of the art IDEs offer this (VSCode, IntelliJ, VS)
• It greatly increases efficiency using the diff panel

Why is the PR from #52935 not enough?

This PR builds on its approach. Thanks to @jacobbinnie. But I noticed a few things:

• never clears the selection when the active repository changes
• a failed checkout or trash disappears silently
• hardcodes single-click = project diff and double-click = file diff, ignoring the entry_primary_click_action setting
• its context menu still reads "Stage File" / "Discard Changes" (singular)
• discard prompt always says "Discard" even for an all-untracked selection that will actually trash files
• no shift+arrow range extension, no Escape to clear
• it skips directory rows entirely, in tree view you can't act on a folder's subtree at all
• its range_select only ever accumulates, consecutive shift+clicks grow the set and nothing shrinks it
• Uses Vec on marks, linear contains scans -> This PR uses HashSets, relevant for huge repos.
• doesn't ship any tests -> This PR covers the whole range with tests too*

* Tests dominate this PR (~850 lines). The actual feature sits around 400.