Add Eclipse workspace marker model and cross-platform CI

[ORESoftware]

What changed

• add a Java workspace model for multiple Zed package roots
• project issues into Problems-style marker records with .zpkg.toml fallbacks
• add confirmation-gated quick-fix command previews with exact executable, argv, and cwd
• run Java 21 Maven/JUnit on fixed Ubuntu 24.04, macOS 15, and Windows 2025
• pin checkout/setup-java by full commit SHA and disable persisted checkout credentials
• record the hardened workflow properties in machine-readable conformance metadata

Validation

Current review head: 246b46048cced1d1072afd3bedc9849efe3a8a8f.

The implementation + immutable/fixed-runner workflow head immediately preceding the metadata-only conformance update passed CI run 31238416607. The current exact-head rerun is pending/queued in the hosted runner pool at last observation; no test failure has been reported.

Tracking

• Linear: DEN-2508
• Independent certification: zed-pkg-test/zed-pkg-e2e#112, pinned to this exact head
• Remaining native gates: PDE/OSGi bundle, resource listeners, view/quick-fix UI, Eclipse application tests, p2 packaging

[linear-code]

DEN-2508

[ORESoftware]

Independent exact-head certification is green in zed-pkg-test/zed-pkg-e2e#112 / workflow run 31210197200. This PR head e65e8086f328a20adf3f941e1be988d0d757dc0f passed Java 21 Maven/JUnit there, in addition to cross-platform repo-local CI 31209660197. Keeping this draft until review and the PDE/OSGi, Eclipse application, and p2 publication gates are completed.