Select All + Expand All freezes the UI for ~17-25 s on a large library

[Marie-Jean THORAVAL]

With a large library, selecting all items and then expanding freezes the whole UI for tens of seconds -- long enough that Windows offers to kill the process.

AI diagnostics below if useful.

Steps to reproduce:
1. A library with many collapsible items (mine: ~18,000 top-level items, ~40,000 rows when expanded).
2. Click into the items list, Ctrl+A.
3. Expand all (or expand the selection with the keyboard).

Timings on 10.0-beta.26+7c52e3013, Windows 11, after a fresh restart with all plugins disabled and auto-sync off, measured with performance.now() around itemsView.expandAllRows() from a script:

    expandAllRows(), nothing selected:   1.1 s  (17,928 -> 40,149 rows)
    selectAll(), then expandAllRows():  17.2 s

The same pair in my normal session (plugins active) is 2.4 s vs ~25 s, so the ratio is stable at roughly 10-15x. The only difference between the two cases is the selection. The per-row expand path (expanding many selected rows via the keyboard) is far worse still, since it pays per-toggle overhead on top -- effectively minutes.

Where the time goes: TreeSelection.adjustForRowInsertion() in chrome/content/zotero/components/virtualized-table.jsx rebuilds the entire `selected` Set on every insertion:

    adjustForRowInsertion(index, count) {
        let selected = new Set();
        for (let selectedIndex of this.selected) {
            selected.add(selectedIndex > index ? selectedIndex + count : selectedIndex);
        }
        this.selected = selected;
        ...

I counted the calls by wrapping adjustForRowInsertion during the frozen expand (the wrapper added ~0.5 s, so the counts describe the real event):

    adjustForRowInsertion calls:      17,882  (one per expanded container)
    selected Set size at every call:  17,928
    total Set operations:        320,588,496  (= 17,882 x 17,928)

So the expand rebuilds a 17,928-entry Set 17,882 times -- about 320 million Set inserts plus 17,882 fresh 18k-entry Set allocations, all on the main thread. (The call sites came in with the recent selection-remapping work -- itemTree.jsx / collectionTree.jsx calling adjustForRowInsertion on row inserts, #5983.)

Debug ID: D2099989934 -- markers "=== [selectall-expand repro]" bracket the baseline and the frozen expand, in a fresh session with all plugins disabled (the only extension active was the debugging bridge used to drive the script, which does not touch the items tree).