fix(engine): make ragged transport padding objective-safe

[EazyReal]

Purpose

Fork-local review mirror for upstream areal-project/AReaL#1566 and the root of the incomplete-group review stack. Do not merge this PR; the upstream PR is the landing authority.

Summary

• add model-valid transport-only microbatches for ragged distributed schedules
• bypass arbitrary objective weight and loss callbacks for transport rows
• remove transport rows from ordinary and tree-packed forward outputs
• preserve semantic transport markers when packed padding is a no-op

Transport padding is backend scheduling state only. It contributes no loss, gradients, metrics, normalization members, or returned semantic rows.

Exact range

08ab30fc8e0cfee70cc2b246119cd9e0c07ae0ce..b0dbd4c423de3be706a3978fd9d1ced4678298e6

Verification

• 86 focused transport/objective tests passed
• full pre-commit passed
• hosted pre-commit and Ubuntu, macOS, SGLang-extra, and vLLM-extra installation checks passed
• independent exact-range transport, Megatron, and Archon review found no actionable issue

A real multi-GPU pipeline canary is not available on this host. CodeRabbit did not produce a review body for this mirror, so its green status is not counted as review evidence.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: eba48a8c-9f86-4d81-b8be-dad2d777f443

📥 Commits

Reviewing files that changed from the base of the PR and between 08ab30f and b0dbd4c.

📒 Files selected for processing (9)

• areal/engine/core/__init__.py
• areal/engine/core/train_engine.py
• areal/engine/fsdp_engine.py
• areal/engine/megatron_engine.py
• areal/experimental/engine/archon_engine.py
• areal/models/tree_attn/tree.py
• areal/utils/data.py
• tests/test_tree_transport.py
• tests/test_utils.py

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