Perf: hoist orderAfterMerge out of the parent-plugin loop (#271)

[elharo]

Fixes #271

Problem

ModelUtils.mergePluginLists() recomputed ModelUtils.orderAfterMerge(...) and
called childContainer.setPlugins(results) / childContainer.flushPluginMap()
on every iteration of the parent-plugin loop, even though the loop body
already accumulates the merged plugins into assembledPlugins.

orderAfterMerge walks both full plugin lists with results.contains/indexOf
linear scans, making the whole merge O(P³) in the worst case for P parent
plugins plus repeated plugin-map flushes. The ordering result is identical
whether computed once at the end or per-iteration.

Fix

Hoist orderAfterMerge(...) + setPlugins/flushPluginMap out of the loop and
compute them once after the loop finishes, using the final assembledPlugins.

Testing

No new tests (performance-only change). All existing tests pass, including the
failsafe integration tests run with -Prun-its (ITSupplementalArtifact
exercises the supplemental-model merge that uses mergePluginLists), plus
spotless/checkstyle/RAT via mvn verify.