Frees v2/models/ to become the vendored-architecture namespace that mirrors fastvideo/models
(part of making v2 self-contained / able to replace fastvideo). The v2 recipe layer (per-family
card.py/loop.py/program.py + common.py + the build_*_engine re-exports) is the recipe, not the
architectures, so it moves to v2/recipes/. The pure-numpy toy/parity implementations (ToyDiT etc.)
move from v2/models/backend.py to v2/platform/backends/toy.py (alongside cpu.py/accel.py/torch_*).
Mechanical: all imports are absolute, so v2.models.<x> -> v2.recipes.<x> and v2.models.backend ->
v2.platform.backends.toy across v2/ + examples/ (89 files, 178 refs). No behavior change.
CPU mini green (202 passed, 2 skipped); all v2 files compile.
Will Lin
51898f48e9