fix(macos): detect named external displays

[tcconnally]

Problem

On the M5 target, system_profiler SPDisplaysDataType reports the two LG ULTRAGEAR+ displays under a Displays: block but omits both Display Type and Connection Type. The soak launcher therefore wrote external_display_observed=0 for a valid docked-clamshell setup.

Change

• Parse immediate child display names under Displays:.
• Preserve the existing legacy Display Type and Connection Type markers.
• Treat clearly built-in display names as non-external and fail closed when no external display is identified.
• Add regression fixtures for the captured M5 shape, legacy markers, internal-only output, and shell integration.

This is related to #79. It does not close the issue or promote any cell count.

Verification

• python3 tests/run_all.py: PASS
• python3 -m compileall -q hyperwall tests: PASS
• bash -n soak_wall.sh: PASS
• Exact-tree review: PASS
• Captured M5-shaped fixture returns external_display_observed=1.

Native acceptance remains separate and incomplete until the fresh fixed-corpus runs are completed.