fix!: apply ReadRel projection during schema inference

[alexandrefimov]

infer_rel_schema currently returns the full base_schema for a masked read. A mask selecting one field can therefore report all input fields, and emit mappings are evaluated against the wrong columns.

Apply the read projection before emit, following the Read Operator in spec v0.99.0. Recursively project struct fields, list elements, and map values while preserving their nullability and container types. Keep explicit RelRoot.names and the original input schema unchanged. For projected reads, leave correlations inside filter and best_effort_filter offset-based: Read Filtering explicitly places those predicates before projection.

Mask unwrapping is a compatibility choice. The unwrapping section describes singleton unwrapping by default but leaves its serialization as TBD. This change follows Java's type projector: it preserves structs and containers at every level regardless of maintain_singular_struct. For example, selecting only the string from a nested struct<i64, string> keeps a nested struct<string>, even with the flag false. The validator instead requires the flag for a single-field read projection.

Struct fields also follow Java's mask order, including reordered and repeated selections. The same spec page still lists column reordering as an open question, so preserving mask order and duplicates remains a compatibility choice.

Closes #264

BREAKING CHANGE: ReadRel projection masks now change inferred output schemas and the indices seen by emit and parent relations. Invalid mask field indices and selector/type mismatches now raise errors instead of being ignored.