Fix bundled DCCCcore launch from the UI

[tctco]

Summary

• resolve the bundled core executable as DCCCcore on Linux/macOS and DCCCcore.exe on Windows
• use the shared resolver in metric calculation, SUVr, AI decoupling, and the main UI module
• add regression coverage for packaged executable names
• bump the release version to 4.2.5

Root cause

The native executable was renamed from CentiloidCalculator to DCCCcore in v4.0, but the Slicer UI continued to launch the legacy filename. Release packages contain only DCCCcore, so the process failed before calculation started.

Impact

This restores PET metric and AI decoupling launches from packaged DCCCSlicer releases. Tagged 4.x releases from v4.0.0-alpha through v4.2.4 are affected.

Validation

• uvx --from pytest pytest -q localizer/src/tests/test_ui_metric_command_builder.py — 6 passed
• CI-equivalent CLI smoke selection — 14 passed
• git diff --check

Fixes #37