Add Eclipse target-platform compatibility matrix workflow (2024-06 → 2026-03)

[Copilot]

This change adds CI coverage for Eclipse release compatibility by building against multiple target platforms (2024-06 through 2026-03) without source edits. It parameterizes Tycho target resolution so the same codebase can be validated across baseline and newer Eclipse streams.

• Build configuration parameterization

  • Added root Maven properties:
    • target.platform.file (default: target-platform.target)
    • target.resolve.with.ee.constraints (default: true)
  • Wired both into target-platform-configuration so CI can switch target and EE behavior via -D flags.

• Target platform definitions

  • Added targets/*.target files for:
    • 2024-06, 2024-09, 2024-12, 2025-03, 2025-06, 2025-09, 2025-12, 2026-03
  • Each target keeps the same IU set as current baseline (org.eclipse.platform.feature.group, org.eclipse.jdt.feature.group) and only varies release repository URL.

• Baseline target indirection

  • Updated target-platform.target to reference targets/2024-06.target via nested <location type="Target" ...>, keeping baseline target content centralized in targets/.

• New compatibility CI workflow

  • Added .github/workflows/compatibility.yml:
    • Windows matrix job across all 8 targets
    • Maven invocation uses quoted -D args (PowerShell-safe)
    • EE constraint behavior is explicit per matrix entry (true for 2024-*; false for 2025-03+)

Example invocation used by the workflow:

mvn -B clean verify -ntp \
  "-Dtarget.platform.file=targets/${{ matrix.target }}.target" \
  "-Dtarget.resolve.with.ee.constraints=${{ matrix.resolve_with_ee_constraints }}"