refactor(tui): unify mapping-editor focus authority on app-shell

Migrates the last two-panel screen onto appShell() + panel() +
Keymap, and fixes the "weak two-panel focus model" TR.B5 flagged:
activePanel was tracked separately from real OpenTUI focus, and
focusActivePanel() faked focus by swapping selectedBackgroundColor
with no border indicator. Three paths (search ENTER, the / key, the
CSV column picker) moved focus outside that method, so tracked
focus, real focus, and the visual highlight could silently desync.

Replaces it with a single focusPanel(target) authority — the only
place border colour, real focus, and focusTarget change together.
'search' and 'right' both live inside the Schema Fields panel, so
both light its border; only which child owns input focus differs.
All three former desync paths now route through it.

Adds a regression suite proving border colour and real focus stay
in sync across Tab, search-focus, and the search-ENTER handoff.
Requires InputRenderable.blur on the shared test double, added here.

Author: JasonWarrenUK