FE-1545: Adopt the worksheet focus layer in the left sidebar lists and search - #9420

🌟 What is the purpose of this PR?

Stacked on #9418. Adopt the worksheet keyboard-flow layer from #9411 across the left sidebar. The sidebar carried three hand-rolled focus models: the filterable lists tracked a virtual focus index in React state over unfocusable rows with hover-only row menus, the nets list made every row and its invisible delete button a tab stop with no arrow keys, and search hopped focus between input and results through DOM queries while remounting the results on every keystroke.

🔗 Related links

• FE-1545 (internal)
• FE-1538: Add the worksheet keyboard-flow layer #9411 introduced the worksheet layer (FE-1538)
• FE-1544: Rebase the token spreadsheet on the worksheet focus layer #9418 is the layer below in the stack (FE-1544)

🔍 What does this change?

• The filterable lists (nodes, types, differential equations, parameters, entities tree) move to real DOM focus through useFocusStops: each list is one Tab stop, arrows move and select as they move, Shift+Arrow extends the range, and group headers collapse/expand with ArrowLeft/ArrowRight. The virtual focusedIndex state, the row-ref bookkeeping, its scroll effect, and a dead per-row key handler are deleted.
• Row menus and group add buttons become keyboard-reachable: a new RowActionSlot registers the row's trailing button as its second column, so ArrowRight reaches it and ArrowLeft returns. The slots reveal on focus as well as hover.
• The nets list folds onto the same model: one Tab stop, arrow keys, Enter/Space activates a net, Delete removes a subnet, and the delete button is reachable with ArrowRight — it was previously invisible-but-tabbable on every row.
• Search joins the input and its results in one vertical FocusStack at the panel level: ArrowDown in the input hands focus to the list and ArrowUp from the first result hands it back, replacing the closest("[data-panel]")/querySelector focus hops. The key={query} remount of the results is gone, so the list no longer resets on every keystroke.
• useFocusStops.tabIndexFor gains a fallback: when the remembered position's stop disappears (a row deleted or filtered away), the table's tab stop falls back to the first stop instead of vanishing from the tab order. Covered by a new layer test.
• The orphaned clampIndex helper is deleted.
• Intentional behaviour changes: focus highlight is now real :focus styling; Escape clears the selection via the list container (as before) and the editor's global Escape; the nets list's row highlight follows focus; list collapse state and click semantics (plain/Shift/Cmd click) are unchanged.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• modifies an npm-publishable library and I have added a changeset file(s)

📜 Does this require a change to the docs?

The changes in this PR:

• require changes to docs which are made as part of this PR

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

⚠️ Known issues

• Single-letter editor shortcuts (v/h/n/t) still fire while a list row holds focus, as they did before this PR. Settling that globally is a follow-up to the focus-layer adoption.
• The section headers between the lists are not part of the arrow flow yet; that is the shared-chrome adoption (headers, tabs, toolbars) proposed separately.

🐾 Next steps

• Shared chrome (SubViewHeader, tab bars, toolbars) is the next adoption target after this and FE-1544 (internal).

🛡 What tests cover this?

• New filterable-list-sub-view.test.tsx: roving tabindex, select-as-you-move with Shift ranges, ArrowRight to the row menu and back, group collapse/expand with hidden children skipped.
• New layer test in focus-flow.test.tsx for the tabIndexFor fallback after the remembered stop unmounts.
• Existing focus-flow.test.tsx covers the movement contract.

❓ How to test this?

1. yarn dev (or the website demo), open the left sidebar.
2. Tab into a list (one stop), walk rows with arrows and watch the canvas selection follow; Shift+ArrowDown to range-select; ArrowRight to a row's ⋯ menu and Enter to open it.
3. In the entities tree, ArrowLeft/ArrowRight on the group headers, ArrowRight to a group's + button.
4. In the Nets list (subnets enabled): arrows between nets, Enter to switch, Delete on a subnet row, ArrowRight to its delete button, double-click to rename.
5. Cmd+F, type a query, ArrowDown into the results, walk them (selection follows), ArrowUp from the first result back into the input — and note the highlight no longer resets while typing.

The sidebar's visuals are unchanged apart from focus highlighting; existing docs screenshots stay valid.

🤖 Generated with Claude Code