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/dsl/grammar/mod.rs

@@ -962,6 +962,56 @@ mod hint_key_tests {
         // Unknown entry word → None (tier-2 fallback).
         assert_eq!(hint_key_for_input_in_mode("zzz", Mode::Simple), None);
     }
+
+    /// Comprehensiveness gate (ADR-0053 D6): every command form in the
+    /// REGISTRY carries at least one `hint_id`, and each resolves to a
+    /// tier-3 `hint.cmd.<id>` block. `keys.rs` checks referenced keys
+    /// resolve; this checks every command *has* one.
+    #[test]
+    fn every_command_form_has_a_tier3_block() {
+        let cat = crate::friendly::catalog();
+        for (node, _category) in super::REGISTRY {
+            assert!(
+                !node.hint_ids.is_empty(),
+                "command `{}` has no hint_ids (ADR-0053 D6)",
+                node.entry.primary
+            );
+            for id in node.hint_ids {
+                let key = format!("hint.cmd.{id}.what");
+                assert!(
+                    cat.get(&key).is_some(),
+                    "missing tier-3 block `{key}` for command `{}`",
+                    node.entry.primary
+                );
+            }
+        }
+    }
+
+    /// Comprehensiveness gate (ADR-0053 D6): every runtime error class
+    /// `friendly::error_hint_class` can return resolves to a tier-3
+    /// `hint.err.<class>` block. Keep this list in sync with
+    /// `error_hint_class` (its own unit tests pin the outputs).
+    /// Diagnostic classes are deferred (issue #38), so not checked here.
+    #[test]
+    fn every_runtime_error_class_has_a_tier3_block() {
+        let cat = crate::friendly::catalog();
+        let classes = [
+            "unique",
+            "foreign_key.child_side",
+            "foreign_key.parent_side",
+            "not_null",
+            "check",
+            "type_mismatch",
+            "not_found",
+            "already_exists",
+            "generic",
+            "invalid_value",
+        ];
+        for c in classes {
+            let key = format!("hint.err.{c}.what");
+            assert!(cat.get(&key).is_some(), "missing tier-3 error block `{key}`");
+        }
+    }
 }
 
 #[cfg(test)]