refactor: extract VariableDetector base class for variable detectors

[viktorbeck98]

Task

Description

Extracts an abstract VariableDetector base class (common/variable_detector.py,
inheriting CoreDetector) that owns the shared lifecycle (train/detect/
configure/post_train/set_configuration) previously duplicated ~90% across the
five variable-value detectors. Each concrete detector — NewValue, Charset,
ValueRange, BigramFrequency, NewValueCombo — is reduced to a config subclass
plus a few override hooks (_check_variable, _prepare_variables, _event_data_kwargs,
_check_event, _description). All detectors stay in the detectors package so
name-based tracker reconstruction keeps working. Net: +439 / −712 lines.
new_event/random/rule are untouched.

Also fixes latent bugs surfaced by the refactor:

• ValueRangeDetector now registers persistency and preserves the persist config
  across set_configuration (both were previously dropped).
• SingleStabilityTracker no longer starts a throwaway PersistencySaver when
  reconstructing a detector to recover its add_value closure (it would leak a saver
  thread per variable and clobber the real state file under persist).

Behavior-preserving; the only intended behavior changes are the two value_range fixes.

How Has This Been Tested?

• Existing per-detector suites are the safety net and stay green (unchanged pass counts).
• Added tests: value_range persist-registration + persist-preservation, and a TDD
  regression test that reconstruction under persist starts no throwaway saver.
• Full suite: 545 passed. prek gate (mypy --strict, flake8, vulture, docformatter,
  bandit) clean.

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.