Add an undeclared-depends_on addon dependency detector

Split out of the CI-pipeline PR (#820) per the one-change-per-PR review:
this is a standalone feature, not CI infrastructure.

Detects addons that import another addon's module without declaring it in
depends_on (the Mantis-13707 class), by loading each registered module in
an isolated subprocess and classifying the failure. The isolated loads run
in a bounded thread pool (configurable; default scales with CPUs) instead
of one 30s-timeout subprocess per module serially — so the detector's
wall-clock stays bounded on the full ~144-addon set (review finding R-F).

Logic split into tests/addon_dependencies.py (the engine) + its test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: eduralph