feat: reject another variant's fields in the enum's seal, and add [Tag]

[jo16oh]

A payload whose tag is a union passed seal with every variant's fields, and the value kept the ones its variant does not declare:

Command.seal({ _tag: name === "new" ? "New" : "Fail", count: 2, message: "m" });
// → { _tag: "New", count: 2, message: "m" }

• The enum's seal (Shape(payload) on a sealer, Shape.seal(payload) on a companion) rejects a field of another variant, both in an object literal and in an object passed through a variable, where TypeScript's excess property check does not reach
• Tag is now a value as well as a type: the key on an enum companion that holds the two below
• Shape[Tag].match(tag, handlers) builds from a tag that arrives without its payload, exhaustively
• Shape[Tag].key returns the tag field's name, for a schema library's discriminator
• The enums page gains "Build a value", a table of which call fits which input