v0.4.2 — honest demo + starter project

Hotfix on top of v0.4.1's downloadable jar: the demo no longer pretends a
project is connected (a hardcoded path that only existed on the author's
machine), and the Release ships a starter Spring Boot project so anyone
evaluating DisC Studio can complete the flow end-to-end without bringing
their own codebase.

Added

• disc-studio-starter.zip attached to the Release — a minimal Spring
  Boot scaffold (Spring Initializr export, ~55KB) with one com.example.demo
  package and an empty design/ folder. Unzip and point the Studio at it to
  evaluate the full save → run flow without an existing Java project.

Changed

• README leads with "What you need" before Quick start. Three numbered
  requirements (Java 21+ · required; a Java/Spring project · required, with
  the starter as fallback; Claude CLI + plugin · optional) replace the
  parenthetical version note from v0.4.1. The reader knows what they're
  signing up for before downloading.

Fixed

• Demo no longer fakes a connected project. "Load simple demo" /
  "Load complex demo" used to set state.projectPath to a hardcoded
  /Users/mossgu/Downloads/demo and run /api/scan against it, producing
  a red error banner for every user except the author. The demos now seed
  story + participants + sequence only; the header chip stays in "Connect
  project" mode until the user pastes a real path.