Type effect callback payloads - #48

Effect callbacks currently force consumers to repeat an untyped payload shape. Export EffectFailurePayload, EffectSuccessPayload, and SerializedError from the root and core entries, and check the actual SQL/Cloudflare message construction against them. Payloads, normalization, retries, and delivery behavior stay unchanged.

Before:

failTurn(payload: { arguments?: Record<string, unknown> }) {
  const generation = Number(payload.arguments?.generation)
}

After:

failTurn({ arguments: original }: EffectFailurePayload<{ generation: number }>) {
  const generation = original.generation
}

The generics describe the application's declared effect contract; they do not infer or validate an independently registered handler. Ruby's corresponding RBS contract is tracked in cardmagic/solid-objects-ruby#64 and preserves its existing Ruby field names.

Validation:

• pnpm run check and pnpm run format:check passed.
• pnpm test: 376 passed, 21 environment-gated skips; database integration jobs run in CI.
• Cloudflare suite: 45 passed; import/type/bundle dry-run checks passed.
• Build and packaged-consumer smoke test passed, including root/core type imports and negative cases against the installed tarball.
• Initial compiler failure demonstrated missing public exports. Temporary mutations to SQL success, SQL failure, and Cloudflare payload construction each produced a compiler error; restored construction compiles.
• Added complete-envelope runtime coverage for success JSON values, undefined-to-null, absent arguments, retry exhaustion, immediate non-retryable failures, and non-Error throws.

Closes #47.