docs: align InferEdgeEnv portfolio positioning

[hyeokjun32]

Summary

• Added the InferEdgeEnv supporting sidecar boundary to the README pipeline overview.
• Clarified that InferEdgeLab is the validation / decision layer and InferEdgeEnv is the run evidence registry / comparability layer.
• Kept Runtime focused on execution/result export ownership.

Validation

• git diff --check
• bash scripts/smoke_default.sh
• python3 tests/test_manifest_compare_identity.py

Notes

• The first sandboxed run of the manifest identity test failed because results/latest.json could not be written; it passed when rerun with normal file permissions.