build: isolate LegacyGL implementation from core builds

[tritao]

Summary

Separate Coin core scene-management APIs from the optional LegacyGL implementation while preserving the public SoNode ABI.

• Keep the base SoNode::GLRender*() virtual surface stable in every configuration.
• Build LegacyGL action, node, element, bundle, and renderer implementation only when COIN_BUILD_LEGACY_GL_RENDERER=ON.
• Keep SoRenderManager and SoSceneManager usable in LegacyGL-disabled builds.
• Advertise the installed renderer capability through COIN_HAVE_LEGACY_GL_RENDERER.
• Validate the public ABI, core scene-manager behavior, traversal, and installed-package boundary in both configurations.

This PR defines the implementation boundary only. It does not introduce a replacement renderer or change scene semantics.

Validation

• Release builds with LegacyGL enabled and disabled.
• Unit tests for both configurations.
• Standalone find_package(Coin) consumer builds against both installed variants.
• Ubuntu, Windows, and macOS LegacyGL ON/OFF CI matrix.

Stack

Layer 7/18, based on PR #38.