fix: initialize /debug before APIC database reads

[monrog2]

Summary

• initialize the dynamic /debug namespace once before legacy APIC database counter collection
• keep initialization best-effort so the existing per-file reads remain authoritative
• add stateful regression coverage for counters that remain unavailable until /debug is listed

Why

On affected pre-6.1(3a) releases, retrying a direct cat /debug/<apic>/<dme>/mitmocounters/mo does not populate the dynamic namespace. The files become readable only after /debug is enumerated, causing the database-size check to report persistent collection errors even though the counters exist.

Validation

• focused APIC database-size suite: 20 passed
• full local suite: 1,017 passed
• Python compilation and git diff --check: passed
• GitLab integration pipeline 226:
  • Python 2.7 and Python 3.8 unit jobs passed
  • 96 forced-legacy fabric runs produced zero missing-counter collection errors
  • the preflight executed in all 91 runs that reached counter collection, with zero preflight warnings
  • 91 database-size checks passed; the remaining five stopped earlier on the same unrelated APIC API timeout
  • actual-version runs produced 15 passes and one legitimate 7.5 GB shard finding

The overall integration pipeline remained red due to existing fabric findings and connectivity/API failures unrelated to this change.

Fixes #446