feat(hint): H2 Phase C batch 5 — runtime error-class tier-3 hints (ADR-0053)

what/example(fix recipe)/concept for the 9 runtime error classes:
foreign_key parent_side (child_side was the exemplar), unique, not_null,
check, type_mismatch, not_found, already_exists, generic, invalid_value.
Keyed by friendly::error_hint_class; catalogue + keys.rs registered.
+1 spot test; 2496 pass / 1 ignored, clippy clean.

src/friendly/keys.rs

@@ -234,6 +234,32 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("hint.err.foreign_key.child_side.what", &[]),
     ("hint.err.foreign_key.child_side.example", &[]),
     ("hint.err.foreign_key.child_side.concept", &[]),
+    // Phase C batch 5 — runtime error-class hints.
+    ("hint.err.foreign_key.parent_side.what", &[]),
+    ("hint.err.foreign_key.parent_side.example", &[]),
+    ("hint.err.foreign_key.parent_side.concept", &[]),
+    ("hint.err.unique.what", &[]),
+    ("hint.err.unique.example", &[]),
+    ("hint.err.unique.concept", &[]),
+    ("hint.err.not_null.what", &[]),
+    ("hint.err.not_null.example", &[]),
+    ("hint.err.not_null.concept", &[]),
+    ("hint.err.check.what", &[]),
+    ("hint.err.check.example", &[]),
+    ("hint.err.check.concept", &[]),
+    ("hint.err.type_mismatch.what", &[]),
+    ("hint.err.type_mismatch.example", &[]),
+    ("hint.err.type_mismatch.concept", &[]),
+    ("hint.err.not_found.what", &[]),
+    ("hint.err.not_found.example", &[]),
+    ("hint.err.not_found.concept", &[]),
+    ("hint.err.already_exists.what", &[]),
+    ("hint.err.already_exists.example", &[]),
+    ("hint.err.already_exists.concept", &[]),
+    ("hint.err.generic.what", &[]),
+    ("hint.err.generic.example", &[]),
+    ("hint.err.invalid_value.what", &[]),
+    ("hint.err.invalid_value.example", &[]),
     // Phase C batch 1 — app-lifecycle command hints.
     ("hint.cmd.quit.what", &[]),
     ("hint.cmd.quit.example", &[]),