Add -enumStyle option for erasable enum code generation

[corbanbrook]

Summary

Adds a configurable -enumStyle generator option so users can choose how webrpc schema
enums (and the generated error enums) are code-generated in TypeScript:

-enumStyle value	Output
enum (default)	Traditional TypeScript export enum — unchanged from current behavior
union	as const object + union type (erasable, no runtime enum)

The option defaults to enum, so existing output is unchanged unless -enumStyle=union
is explicitly passed.

Motivation

TypeScript best practices are moving away from enum declarations. They emit runtime
code that cannot be erased, which is incompatible with the erasableSyntaxOnly
tsconfig option and with tooling (Node's type stripping, modern bundlers) that expects
type-only syntax to disappear at build time. The union style emits a plain const
object plus a derived union type, leaving no non-erasable runtime construct behind.

Output comparison

Given this schema:

enum Kind: uint32
  - USER
  - ADMIN

enum Intent: string
  - openSession
  - closeSession

-enumStyle=enum (default)

export enum Kind {
  USER = 'USER',
  ADMIN = 'ADMIN'
}

-enumStyle=union

export const Kind = {
  USER: 'USER',
  ADMIN: 'ADMIN',
} as const
export type Kind = (typeof Kind)[keyof typeof Kind]

In union mode the same treatment is applied to the generated errors and
WebrpcErrorCodes enums, so the entire generated file is free of runtime enum
declarations and passes tsc --erasableSyntaxOnly.

Caveats

The union style is mostly a drop-in replacement for an enum, with two behavioral
differences for consumer code:

• No reverse mapping for numeric enums. WebrpcErrorCodes[1000] returns undefined
  (a numeric enum would return 'Unauthorized'). Code mapping an error code back to its
  name must be updated. Schema enums are unaffected (they use string values).
• Members can't be used directly as types. type T = Kind.USER becomes
  type T = typeof Kind.USER.

Changes

File	Change
main.go.tmpl	Add -enumStyle option defaulting to enum; validate value is enum or union, otherwise print an error and exit
types.go.tmpl	Branch on $opts.enumStyle — existing export enum block for enum, new as const + union block for union
errors.go.tmpl	Apply the same union treatment to the errors and WebrpcErrorCodes enums so union output is fully erasable
README.md	Document the option, the output comparison, and the caveats above
tests-unit/	New enum.ridl schema + golden fixtures (enum-default.gen.ts, enum-union.gen.ts) and a enum-union.test.ts vitest spec
.github/workflows/ci.yml	Regenerate the enum fixtures from current templates and run the tests-unit vitest suite on every PR

Testing

tests-unit/enum-union.test.ts verifies:

• Schema and error enums expose the correct runtime values in union mode
• The union name works in both value and type position (drop-in usage)
• The union output contains no runtime export enum, and the default output still does
• tsc --erasableSyntaxOnly compiles the union fixture clean and rejects the default
  fixture with TS1294

Invalid values are rejected: -enumStyle=foo →
-enumStyle="foo" is not supported, must be "enum" or "union".

[AlexanderKolberg]

This is great!
I really like this approach with the as const obj + union

[AlexanderKolberg]

LGTM!