Merge the fixture ecosystems and restructure the test suites

[AlexSkrypnyk]

3 fixture ecosystems become 1, and the test tree gains the kernel suite the driver brings along.

• 1 fixture module set on 1 scaffolded site replaces driver_field_test (driver), behat_test (extension) and the v3 fixture site + mysite_core.
• tests/Unit/ (driver units, field handlers, parser, trait logic), tests/Kernel/ (the SQLite kernel suite including the field-type coverage safety net), tests/Behat/ (blackbox profile on phpserver-served fixtures, drupal profile on the fixture site, the behat-in-behat CLI harness proving each trait in isolation).
• The cross-repo smoke jobs and their path-repository version-shape detection die - the whole stack tests in-tree on every commit.
• The self-test suites split per surface (blackbox / api / javascript) and become the reference layout the docs teach.