v0.4.1 — downloadable jar

DisC Studio is now downloadable. Grab disc-studio-0.4.1.jar from the
Release page, java -jar, open localhost:8080 — no clone or Gradle
required.

Added

• Downloadable jar. ./gradlew bootJar produces a single fat jar
  (disc-studio-0.4.1.jar, ~30MB) attached to the GitHub Release. Run
  with java -jar disc-studio-0.4.1.jar on any machine with Java 21+.

Changed

• README restructured around "download-and-run" first. New Quick
  start section leads with the jar path. Running from source demoted
  into a Development section for contributors.

Fixed

• Composer step number off-by-N. The "Add step N" composer was
  counting fragment markers (loop, else, end) and SUT boundary
  edges ([*] -> SUT, [*] <-- SUT) as steps, so "Step 9" would
  appear above a list whose last CALL badge read "7". Now counts CALL
  rows only, matching the badges.

Removed

• Dead /api/disc-steps endpoint. The Java controller, the
  loadDiscSteps() JS indirection, and the on-disk skill bundle were
  serving a path that no longer had a frontend consumer. The run
  checklist now reads from a hardcoded DISC_STEPS constant in
  app.js (the run-event regex matches step numbers only, so a title
  mismatch couldn't break event mapping anyway). Net –80 lines.