Yeah! After a few tweaks, I was able to get my application to launch without obvious errors.
A followup to close the loop for other tycho users.
I think the <environment> and <launcherArgs> in the .target file were installed by the Eclipse Target editor. I deleted these elements, with no affect on the build. It didn't hurt, but neither did it help with the NoClassDefFoundError for SWTError.
It took me a while to find the "validate" button on the Product Configuration (PC) editor. It is an icon in the upper right corner that resembles a text file with an overlayed check mark. Once I got a clue regarding which components were missing, there were still some dots to connect.
The one helpful step was using the PC editor's Add Required button. This seems to pull in the transitive closure of included features. Although the editor page suggests that "Nested features need not be listed", listing the entire transitive tree of features seemed to be essential.
For each missing plugin reported by the PC editor's validate report indicated a missing plugin, I used the Feature Manifest editor to add an org.eclipse feature to my bundling feature (...platform). I wound up "including" the following org.eclipse features in my platform feature:
org.eclipse.e4.rcp
org.eclipse.emf.common
org.eclipse.emf.ecore
org.eclipse.rcp
This seems like the right minimal set, but the I'd love to see a concise list and explanation of the minimal set of RCP plugins, fragments, and features. Once these were "included" in my platform feature, the PC editor's Add Required button added everything to the product's dependency list (i.e. <feature> element in the .product file).