Track migrating rcs_taxim to the current scene/config stack #288

Context

PR #268 adds rcs_taxim, but its integration still relies on a legacy custom sim/env path and custom scene files from the pre-config/pre-scene-stack era.

For the PR refresh, we want to keep the change set focused on making #268 work again on current master and getting CI green. The broader architectural migration should happen separately.

Follow-up

Port rcs_taxim onto the current RCS simulation/config stack:

• replace the legacy custom env creator path with the current RCSEnvCreator / SimEnvCreatorConfig flow where practical
• decide whether the custom TAXIM FR3 scene should stay as a dedicated MJCF scene or be rebuilt via the scene/composer/task abstractions
• update the example(s) to use the modern config-based env setup
• keep the extension decoupled from removed/deprecated pre-refactor APIs

Why this matters

Right now the old integration shape makes rebases against current master harder than they should be and mixes architectural migration with PR #268’s functional goal.