v1.5.3

Closes the "Space surfaces what's possible here" loop. v1.5.1 wired it
on panel 2 and panel 3; v1.5.2 added it on container drill; v1.5.3
brings it to panel 1 too, so the rule is now universal: anywhere the
user can land focus, Space shows what they can do.

Added

• Panel 1 (sidebar) Space cheatsheet. Sidebar rows are nav targets,
  not action targets — so a per-row menu wouldn't make sense. Instead
  Space opens a read-only popup listing the keys that drive the sidebar
  (j/k move, Enter focus, / search, search-mode Enter to lock, Esc to
  clear, N/C global pickers). Search-mode Enter/Esc are visually nested
  under / with the same drill-into arrow used in the Relatives tab.
  Esc/q/Space/Enter all close. Long hints wrap onto continuation lines
  with the key column left empty.

Changed

• Space scope in README now spelled out per panel. Three-key blurb
  • Key Bindings table both updated to enumerate which popup opens on
    which panel (sidebar cheatsheet / panel 2 menu / Relatives breadcrumb).