Tasks: Palette usability

• T001 Record clarified spec and plan.
• T002 [US1] Add and run a red native long-list keyboard regression.
• T003 [US1] Reveal keyboard and filter selection without stealing search focus.
• T004 [US2] Add red no-match, recovery, accelerator and narrow-layout assertions.
• T005 [US2] Add native empty feedback and shortcut hints.
• T006 Inspect native light/dark, narrow and enlarged high-contrast renders.
• T007 Resolve Grok review, update docs and feature map, run full verification.
• T008 Commit, publish PR and link it to the thread.

Dependencies: T002 precedes T003; T004 precedes T005; both feed T006/T007 then T008. Parent is the sole writer; Grok is the independent read-only reviewer.

Published PR: https://github.com/Lucenx9/signaltty/pull/39.