[refactor] v2: use absolute imports (v2.*) everywhere instead of relative

Mechanical conversion of every relative import under v2/ to an absolute v2.* path
(from .x / ..x / ...x -> from v2.<pkg>.x) so imports are unambiguous, grep-able, and
stable when code is copied/moved between entrypoints (VideoGenerator, CLI, server).

Surgical prefix-only rewrite: only the 'from <dots><module>' prefix changed — import
names, parentheses, multi-line formatting, comments, and ordering are byte-for-byte
preserved (no collapsing, no reorder, no unrelated reformatting).

- 471 imports across 125 files; v2/tests/ was already absolute (untouched).
- Validated: all 125 files compile, every 'from v2.* import' target resolves to a real
  module/package, zero relative imports remain (full sweep), CPU mini suite green
  (202 passed, 2 skipped).

v2/program/workflow.py

@@ -111,7 +111,7 @@ class ParallelWorkflow(Workflow):
     def run(self, engine: Any, **initial: Any) -> Any:
         if not self.stages:
             raise ValueError(f"workflow {self.workflow_id!r} has no stages")
-        from ..request.artifacts import Output
+        from v2.request.artifacts import Output
         merged: dict[str, Any] = {}
         metrics: dict[str, float] = {}
         rid = self.workflow_id