perf: default CONTEXT_LEN to 16k (~2.5x faster CPU inference; NTV3_CONTEXT_LEN overrides)

feat: make CONTEXT_LEN env-configurable via NTV3_CONTEXT_LEN (default 32k)

feat(spaces): fast hg38.2bit local lookup with auto-download from HF dataset

fix(spaces): use Ensembl REST as fallback for human (HF egress blocks UCSC)

debug: log UCSC/Ensembl fetch failures + surface them in UI

Initial deploy: MAGI variant interpreter (gradio_app)