Extend the current TERPSICHORE-native external-region implementation so it can
correctly support the projection_wall geometry mode, not just the simpler wall
construction already present in the reduced source snapshot.
Inspect the source snapshot and run the public reproduction, then complete the implementation so projection-wall geometry changes propagate through the pseudo-vacuum construction and into downstream external-region scientific payloads.
Use the supported projection_wall configuration and observed behavior to
trace the representation through the external-region implementation.
For this task, projection_wall should be treated as a wall-construction
regime rather than a cosmetic parameter tweak. The constructed outer geometry
must remain compatible with the existing downstream external-region machinery;
it should not introduce a disconnected one-off path that only satisfies a
single reproduction case.
Constraints:
- keep the solution generic and array-based;
- do not hard-code the public reproduction values;
- preserve the simpler wall mode already supported by the snapshot;
- keep the public reproduction runnable without internet access.