Distributed create/overwrite writes assign Lance field ids positionally, so
blocks whose columns arrive in a different order (e.g. a union of differently
ordered sources) were committed under a single schema and read back transposed.
Add a guard in LanceDatasink.on_write_complete that raises on inconsistent
column order, and align Arrow blocks by name in pd_to_arrow so an explicit
schema writes correctly.
Also harden the schema-evolution mode plumbing: document per-mode schema
behaviour (append validates/drops; create and overwrite evolve the schema),
fix the LanceDatasink default-mode docstring, document the write_fragment mode
parameter and the writer/committer mode-pairing requirement, and add regression
tests for multi-fragment overwrite, schema drop/type-change, streaming overwrite
resume, streaming-append rejection, and the column-order guard.