feat(hint): H2 Phase D — coverage gate, F1 strip, status flips (ADR-0053)

Completes H2:
- comprehensiveness coverage tests: every REGISTRY command form has a
  hint_id resolving to a hint.cmd.* block, and every runtime error class
  resolves to a hint.err.* block (enforces ADR-0053 D6)
- ADR-0051 keybinding strip advertises F1 in the editing (leads) and
  default states; +shortcut.hint label; 12 full-panel snapshots
  re-accepted (status-bar line only)
- flip ADR-0053 -> implemented, requirements H2 + A1 -> [x], README

2498 pass / 1 ignored, clippy clean.

src/friendly/strings/en-US.yaml

@@ -1165,6 +1165,7 @@ shortcut:
   browse: "browse"
   clear: "clear"
   complete: "complete"
+  hint: "hint"
   history: "history"
   home_end: "home/end"
   del_word: "del word"