FlowDrop 2.0.0

The 2.x line is stable. The public API is frozen from this tag: all 679 in-scope
production classes carry exactly one of @api or @internal, and the BC policy's
promise starts here.

Known defect, documented in docs/reference/api/bc-policy.md: OrchestratorInterface
is @api but is typed entirely in @internal DTOs. Until 3.0.0 resolves it, the DTOs
reached from OrchestrationRequest are treated as covered by the @api promise; anyone
needing it resolved should open an issue and contact the maintainers.

Gate at this tag: phpcs, phpstan level 8 (1066 files, zero production baseline) and
the full phpunit-parallel suite (325 classes) green. Playwright e2e NOT run — the
bundled editor moved 2.0.0-beta.11 -> 2.0.0 and that path is unverified here.