Select the execution root as Maven's current project

Lifecycle extensions could receive a dependency module as the current project when invoked at a multimodule execution root, leading to incorrect module selection.

Chooses the selected execution root while retaining the first-project fallback for filtered reactors. Reactor order, per-module overrides, null clearing, thread-local state, and cloned-session behavior are preserved.

• Gives lifecycle participants the correct initial project context in multimodule builds.
• Corrects extension-visible selection without reordering reactor execution or removing filtered-reactor fallback.

• The PR reports failing-before/passing-after regressions and the original reproducer selecting parent instead of child1 on JDK 21.
• Full verify and Core IT runs passed; GitHub build/integration matrices passed on Linux, macOS, Windows and JDK 17/21/25. The issue is closed and the verified merge commit credits Goutam Adwant.