feat: add java calculator example

[b3cramer]

Demo branch for Aspect Workflows + Buildkite integration showcase.

Adds a java/ package with a simple Calculator library and full JUnit test suite — clean baseline for demoing the build/test pipeline.

The demo break commit on this branch introduces a bug (multiply returns a + b instead of a * b) to show how the Buildkite test step catches regressions.

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Wed May 20 20:33:44 UTC 2026

✅ 6 successful tasks

• ✅ format (buildifier-gha) · ⏱ 8.6s · ✨ Aspect · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ delivery (delivery-gha) · ⏱ 9.5s · 🐙 GitHub Actions · ☑️ Check
  💬 Delivery complete (no deliveries)
• ✅ format (format-gha) · ⏱ 14.2s · ✨ Aspect · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ gazelle (gazelle-gha) · ⏱ 13.5s · ✨ Aspect · 🐙 GitHub Actions · ☑️ Check
  💬 Gazelle complete (clean)
• ✅ lint (lint-gha) · ⏱ 30.9s · ✨ Aspect · 🐙 GitHub Actions · ☑️ Check
  💬 Lint complete (clean)
• ✅ test (test-gha) · ⏱ 15s · ✨ Aspect · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (33/33 passed · 32 cached)

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