build: isolate LegacyGL implementation from core builds

[tritao]

Summary

Establish the source and public API boundary that makes the LegacyGL renderer genuinely optional.

• Exclude LegacyGL-only implementation units from core-only builds rather than compiling them into inert code.
• Gate corresponding class registration and amalgamation includes.
• Use the installed `COIN_HAVE_LEGACY_GL_RENDERER` capability for LegacyGL-only public API availability.
• Keep `SoRenderManager` and `SoSceneManager` available when LegacyGL is disabled.
• Make their basic lifetime, viewport, and shared state independent of `SoGLRenderAction`.
• Add installed-package capability checks and manager coverage for Legacy-renderer-disabled builds.
• Establish the final Ubuntu, Windows, and macOS Legacy renderer ON/OFF CI matrix.

This layer creates the architectural boundary only. It does not introduce the retained DrawList renderer.

Stack

Layer 7/18, based on PR #38.