SRJ-23 sample 002: restore missing topology for disconnected target regions

[0hmX]

Parent: SRJ-23 remaining completion failures.

Problem

Pipeline 7 cannot route four connections because their endpoint capacity regions have no graph path to the surrounding mesh:

• source_net_3_mst0: pcb_port_16 -> pcb_port_8
• source_net_3_mst1: pcb_port_16 -> pcb_port_3
• source_net_2_mst0: pcb_port_17 -> pcb_port_2
• source_net_2_mst1: pcb_port_2 -> pcb_port_50

The affected target regions are cmn_290 at pcb_port_16 and cmn_289 at pcb_port_2. Exhaustive searches remain disconnected even when region reservations, endpoint reservations, section masks, and preferred-start selection are ignored.

Root-cause area

Topology/edge generation drops the non-terminal connection between these target regions and the global mesh. This is not congestion.

Acceptance criteria

• All four connections have a valid non-terminal graph path.
• SRJ-23 sample 002 completes under the normal benchmark configuration.
• Add a focused regression test that fails if either target region becomes graph-isolated.
• Do not add a fallback or relax unrelated routing constraints.