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/strings/en-US.yaml

@@ -592,11 +592,48 @@ hint:
       example: "explain select * from Customers where email = 'a@example.io'"
       concept: "Like simple mode's `explain`, but wraps a raw SQL statement. It reveals whether an index is used, and never executes."
   err:
+    # Runtime error classes (Phase C batch 5), keyed by
+    # friendly::error_hint_class. `example` is a fix recipe rather than a
+    # runnable line; `concept` is the relational idea behind the rule.
     foreign_key:
       child_side:
         what: "The value you gave for the child column doesn't match any parent row, so the foreign key has nothing to point at."
         example: "First insert the parent (insert into Customers …), then the child that references it."
         concept: "A foreign key is a promise that every child points at a real parent, so the parent must exist first. To allow orphans on delete instead, set the relationship's `on delete` to `set null` or `cascade`."
+      parent_side:
+        what: "You're deleting or changing a row that other rows point at, which would orphan those children."
+        example: "Delete the child rows first, or set the relationship's `on delete` to `cascade` (remove them too) or `set null` (keep them, unlinked)."
+        concept: "A foreign key guarantees every child has a real parent, so the database won't remove a parent out from under its children unless the relationship says what should happen to them."
+    unique:
+      what: "A value you're inserting — or updating to — already exists in a column that must be unique."
+      example: "Pick a different value, or update the existing row instead of inserting a new one."
+      concept: "A unique constraint (and every primary key) forbids duplicates, so each value identifies at most one row."
+    not_null:
+      what: "You left a column empty that is required to have a value."
+      example: "Supply a value for the column, or give it a default so new rows fill it automatically."
+      concept: "A `not null` constraint means every row must have a value there — it's how you mark a fact as mandatory."
+    check:
+      what: "A value broke a `check` rule defined on the column."
+      example: "Use a value the rule allows — for example a positive number, or one of the permitted options."
+      concept: "A `check` constraint is a condition every row must satisfy, so the database enforces business rules like \"price ≥ 0\" for you."
+    type_mismatch:
+      what: "A value doesn't fit the column's type — for instance text where a number is expected."
+      example: "Give a value of the right type: a number for `int`/`real`, a quoted string for `text`, true/false for `bool`."
+      concept: "Every column has a type, and the database rejects values that don't fit, so a column's data stays consistent and comparable."
+    not_found:
+      what: "You named a table or column that doesn't exist."
+      example: "Check the spelling, or run `show tables` (or `show table <name>`) to see what's there."
+      concept: "A command can only refer to tables and columns that already exist — create them first if you need them."
+    already_exists:
+      what: "You tried to create a table, column, relationship, or index whose name is already taken."
+      example: "Pick a different name, or drop the existing one first if you meant to replace it."
+      concept: "Names must be unique within their kind so a command is never ambiguous about what it refers to."
+    generic:
+      what: "The database refused the command for the reason shown above."
+      example: "Read that message for the specifics, adjust the command, and try again."
+    invalid_value:
+      what: "A value or option in the command wasn't valid for where it was used."
+      example: "Check the value against the column's type and the command's accepted options."
   # Invalid identifier in a schema slot (ADR-0022 stage 8e
   # + the user's #5). Voice mirrors ADR-0019's "no such
   # {kind}" wording for consistency with engine errors.