refactor(fastsac): migrate SONIC to PolicyVariant API

[Jiaxi-Huang]

Summary

Migrate the SONIC-specific FastSAC integration to the neutral PolicyVariant contract, while preserving the generic FastSAC core.

Baseline:

• perf(fastsac): async collector push 的拷贝冗余与同步阻塞 #61/perf(fastsac): async learner 进程抢 CPU 核，拖慢 collector 的 env.step #62 are already merged through perf(fastsac): async collector push 的拷贝冗余与同步阻塞 #63 into origin/main.
• The current sonic branch is based on perf(fastsac): async collector push 的拷贝冗余与同步阻塞 #63 and adds the SONIC G1 workflow.
• Preserve perf(fastsac): async collector push 的拷贝冗余与同步阻塞 #63 behavior: 6-field transition ring, successor-slot next_obs, ring_capacity >= 2, and learner/collector CPU affinity.

Goals

• Add the neutral PolicyVariant contract and registry in motrix_rl/fastsac/api.py.
• Register default and sonic; SONIC registration happens only through the documented side-effect import in motrix_rl/fastsac/__init__.py.
• Replace algo.sonic with algo.policy_variant plus the opaque algo.variant mapping.
• Remove FastSAC-core sonic_cfg branches and SONIC imports.
• Checkpoints save policy_variant and self-describing policy_variant_metadata; export rebuilds actors from that metadata.
• Remove profile-specific SONIC environments and retain only g1-sonic, using one task recipe and one canonical PolicyVariant config with 10 future frames.
• Keep compact observation reconstruction, GPU-env async D2H staging, deployment format changes, and external official SONIC release-loader behavior out of scope.

Compatibility

• Existing default FastSAC checkpoints without variant metadata remain exportable.
• Checkpoints from the pre-merge SONIC branch using "sonic": {"enabled": ...} are not supported and should fail with a clear message.
• g1-sonic-lafan and g1-sonic-smoke registrations and task recipes are removed in favor of the single g1-sonic environment.

Acceptance

• Targeted tests covering policy variants, SONIC actor/aux semantics, collector, learner, ONNX export, async workers, release loading, and SONIC environment/motion/manager contracts.
• Full pytest suite, pre-commit hooks, and whitespace checks.
• Documentation updated bilingually and generated environment overview checked.
• Static scans confirm no SONIC imports or branches in generic FastSAC modules and no obsolete environment/profile config references.