feat: add TrainerRank

[bradhilton]

Summary

• add art.megatron.trainer_rank plus a minimal dev/trainer_rank.py torchrun demo
• add shared-prefix packing/tree helpers and unify GDN execution around the generic tree path
• add TrainerRank request-head support for target logprobs, multi-target labels, top-k, logits, and hidden states
• add topology/perf/parity dev harnesses and unit/integration coverage

Validation

• uv run ruff check src/art/megatron/context_parallel/builder.py src/art/megatron/shared_prefix_state.py tests/unit/test_shared_prefix_attention_builder.py dev/trainer_rank_perf.py
• uv run pytest tests/unit/test_shared_prefix_packing.py tests/unit/test_shared_prefix_tree.py tests/unit/test_shared_prefix_grad_parity.py tests/unit/test_trainer_rank_validation.py tests/unit/test_shared_prefix_attention_builder.py (34 passed, 8 skipped locally; Megatron-only attention builder passes on H200)
• H200: shared-prefix attention builder 7 passed
• H200: GDN CP packed correctness 7 passed, 2 skipped
• H200: real GDN/native FLA CP 4 passed, 2 skipped
• H200: Qwen35 full-model CP1 packed vs flattened 1 passed
• H200: TrainerRank topology matrix 120/120 passed across DP/TP/CP <= 4 and depths 0..3
• H200 35B/A3B CP=4 EP=4 perf guards: Austin 198k, depth-3 random, no-sharing 90k, mixed hidden/logits/top-k outputs