[selmanozleyen]

Drops the annbatch shim (vendored fork samplers plus a monkeypatched Loader.__iter__) and the annbatch==0.2.1 pin.

• annbatch>=0.3.1: it ships the #256 multi-dataset fix and a reworked ClassSampler (with RLEManager).
• The primary samples with annbatch's ClassSampler.
• Links use a new BoundClassSampler (scfit/data/_bound_sampler.py), a ClassSampler subclass. For each batch of the primary's schedule it draws one of its own classes matching on match_on, ∝ its weights within that context, and reads whole chunks of it through annbatch's RLEManager. An inner class with no positive-weight match is refused at construction.
• The per-batch group comes from obs instead of the patched batch["label"]: each per-rep loader carries every row's leaf as an obs column, and a batch's leaf is its first row's. A link now reports its own leaf directly (it no longer carries the match_on prefix).

All existing data tests pass unchanged against annbatch 0.3.1, plus two for BoundClassSampler.