Message type generator: restore Dates and nested class instances without decorators #294

JsonSerializer loses runtime types: Dates arrive as strings and nested objects as plain objects. The current answer, bus-class-serializer, relies on class-transformer (unmaintained since 2021), needs legacy experimentalDecorators/emitDecoratorMetadata (not emitted by esbuild, tsx, Vite or TC39 decorators), a global reflect-metadata polyfill, calls constructors with no args, and fails silently on a forgotten @Type. Schema-first libraries were ruled out because messages should stay plain TypeScript.

Proposal

• A bus generate-message-types command (shares the CLI package with CLI: apply and prune transport topology (queues, topics, subscriptions) #283 and Project scaffolding: npm create bus-app templates #269) reads the message library's source with the TypeScript compiler API, read-only like tsc --noEmit, and emits a plain .ts file (e.g. src/message-types.generated.ts) mapping each $name to the runtime type of every nested path: Date, Map, Set, BigInt, nested classes, arrays, optionals. No transformer or bundler plugin, so it compiles with any toolchain.
• The default serializer uses the map to revive Dates and class instances at any depth. Without a map it still restores the top-level class from $name.
• Unsupported types (functions, unresolvable types) fail generation; --check for CI; bus startup throws if a handled message has no map entry.
• Plain-data messages (ISO date strings) stay the documented zero-setup option, and the path for plain JS projects.
• Deprecate and remove bus-class-serializer, dropping reflect-metadata and class-transformer from the ecosystem. Breaking, so ship as minor with a Breaking: note and a MIGRATING.md entry.

Scope

• Generator command plus --check
• Revival map format and serializer support (nested, arrays, optionals, Map, Set, BigInt)
• Startup validation for messages missing from the map
• Tests, including a round-trip conformance case in bus-test (replacing the ClassSerializer round-trip test from build bus-test to dist and fix its dependency types #292)
• Docs: messages guide, plain-data guidance, migration from bus-class-serializer
• Remove bus-class-serializer and its reflect-metadata/class-transformer deps everywhere

Known limits, to be documented: revived objects are created from the prototype without running constructors, and #private fields aren't restored.

Related: #283, #269, #263.