[turinglambdaai]

Problem

Linux Taskly uses GTK_SELECTION_NONE for its task list despite the cross-platform PRODUCT-SPEC requiring row selection, Shift/Ctrl selection, Up/Down navigation, Return to expand and Esc to clear. Double-click editing used a custom gesture instead of the native GTK row-activated signal, so keyboard activation could not work.

Change

• Enable GTK_SELECTION_MULTIPLE (GTK-native Ctrl/Shift click selection and keyboard focus/range selection).
• Use native row-activated on Return/double click to toggle the inline editor; disable single-click activation so a normal click only selects a row. Remove the competing custom task double-click handler to prevent double toggles. Keep list editing double-click and right-click context menus as before.
• Preserve selected task IDs across periodic snapshot-driven row rebuilds (do not reset selection every poll).
• Extend the existing Esc ladder: editor → search → clear task selection, without hijacking Esc when nothing is selected.
• Update UX benchmark status, explicitly retaining bulk actions as a separate incomplete item.

QA before merging

• Linux GTK host build + packaged Product Acceptance succeed.
• Click row only selects; double-click / Enter opens editor exactly once.
• Ctrl-click toggles, Shift-click selects a range, Up/Down navigates, Esc clears after editor closes.
• Selection persists across 2s snapshot refresh; disappears naturally when its task leaves the view.
• Checkbox, task context menu, completed header and list-name double-click still work.
• Search and inline text entry continue to own their keys.

Scope: This is a focused Linux parity step, not full interaction parity: bulk completion, move, delete and Windows multi-selection remain for follow-up PRs. Do not merge or release until all checks are green.