Delta schema mismatch when column renames change target names

[malon64]

When column renames change target names (columns[].source + columns[].name), Delta writes fail if the table already exists with a different schema. Example error: Schema error: Field customerid not found in schema. Today the workaround is to delete the delta table. We should consider a schema evolution/merge strategy or a clearer validation error in a future release.

[malon64]

Backlog cleanup note: schema-evolution roadmap planning (including columns[].source/name rename scenarios) was initially split into #179, which is now closed and folded back into this issue to keep the backlog grouped by theme.