refactor: replace FSDP with FSDP2 as default parallel backend

- Remove FSDPExecutor (FullyShardedDataParallel wrapper)
- Rename FSDP2Executor to FSDPExecutor, register as 'fsdp'
- Remove 'fsdp2' from CLI choices, make 'fsdp' the default parallel_mode
- Pass after_wrap to executor.prepare for compile-after-wrap ordering
- Update architecture.md, params.md, AGENTS.md references
- FSDP2 uses per-module fully_shard: no FlatParameter, better compile compat

astrai/parallel/__init__.py

@@ -4,7 +4,6 @@ from astrai.parallel.executor import (
     BaseExecutor,
     DDPExecutor,
     ExecutorFactory,
-    FSDP2Executor,
     FSDPExecutor,
     GradientState,
     NoneExecutor,
@@ -36,5 +35,4 @@ __all__ = [
     "NoneExecutor",
     "DDPExecutor",
     "FSDPExecutor",
-    "FSDP2Executor",
 ]