feat(streampetr): harden the streaming training infrastructure

[vividf]

Summary

Hardens the training infrastructure that the production StreamPETR recipes rely on. Four independent pieces, grouped because they are all training-loop plumbing.

Changes

• Partial-annotation status plumbing (datamodule/common/multiview_detection3d.py + subclasses): a configurable annotation_status_field names the per-frame annotation-completeness field in the info files; samples expose it under the generic annotation_status key. Nothing dataset-specific is hardcoded in the shared layer, and datasets without the field emit no key at all.
• Scene-contiguity validation: prev_exists is derived from file-order adjacency, so an annotation file that interleaves scenes would silently reset temporal memory mid-scene — dataset construction now fails loudly instead.
• Checkpoint loading (utils/checkpoints.py): shared-tensor alias detection (identity by storage) so modules registered twice — e.g. a backbone reachable both directly and through a feature extractor — pass full-coverage checks; shape mismatches still refuse to load with a per-key report.
• Epoch-end validation loop (utils/lightning_loops.py): Lightning derives val_check_batch once from the dataloader length at setup; variable-length samplers (the streaming sampler trims differently per epoch) make shorter epochs silently skip validation, feeding stale metrics to the checkpoint monitor. The override triggers validation on the true last batch of a due epoch. Step-based validation configs (check_val_every_n_epoch=None) are left untouched. A canary test signals when the upstream fix lands so the workaround can be retired.
• IterWarmupEpochCosineLR: 500-iteration linear warmup multiplied by per-epoch cosine decay (the mmengine chained-scheduler recipe). Closed-form over the step counter, so checkpoint resume replays the original schedule even when the framework re-estimates total_steps; covered by a resume-trajectory test.

Testing

Unit tests for every piece (serialization dtype round-trips, contiguity guard, alias loading, loop behavior incl. the upstream canary, scheduler shape/boundary/resume). Full suite green at this commit.

Stack 5/8 — base: feat/streampetr-base-configs.