[refactor]: remove VideoGenerator.generate_video

VideoGenerator.generate(request) is the one inference entry point. The
generate_video keyword call, its mapping from flat keywords to a request
(legacy_generate_call_to_request and its helpers), and the tests of that
mapping are removed. Comments and docs that named generate_video describe
generate and request extensions instead.

scripts/ltx2_sr_alignment.py keeps its generate_video call, because its
reference run imports the separate FastVideo-internal package.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012M91wnVFmPEvJ9h39r5BH7

docs/contributing/performance_benchmarks.md

@@ -112,7 +112,7 @@ per-metric policy with direction, percent threshold, absolute threshold, and a
 
 `test_inference_performance.py` temporarily sets `FASTVIDEO_STAGE_LOGGING=1`
 while it runs so pipeline stage execution times are available in
-`generate_video(...).logging_info`. Stage logs use pipeline-unique keys such as
+`generate(...).logging_info`. Stage logs use pipeline-unique keys such as
 `prompt_encoding_stage` so duplicate stage classes do not collide. For
 `PipelineStage` entries, shared component stage bases emit a stable
 `component_metric`: text encoding stages map to `text_encoder_time_s`,