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/snapshots/rdbms_playground__ui__tests__relationships_panel_dark.snap

@@ -1,6 +1,6 @@
 ---
 source: src/ui.rs
-assertion_line: 3209
+assertion_line: 3212
 expression: snapshot
 ---
 ╭ Tables ──────────────────╮╭ Output ────────────────────────────────────────────────────────────────────────╮
@@ -26,4 +26,4 @@ expression: snapshot
 │    Orders.customer_id    ││for SQL                                                                         │
 ╰──────────────────────────╯╰────────────────────────────────────────────────────────────────────────────────╯
 Project: Term Planner                                                                                         
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