Preserve batched Excel writes across recipe batches

[ebhills]

Summary

• prevent recipe batching from writing partial intermediate results
• accumulate repeated Excel sheet writes for append and overwrite modes
• switch subsequent overwrite batches to append so earlier output is retained
• align dynamic or reordered batch columns by column name instead of position
• add focused connector and recipe regression coverage

Why

Recipe batches can produce multiple writes to the same Excel sheet. Previously, intermediate recipe batches could be written independently, overwrite mode could discard earlier batches, and changing dictionary keys could shift values into the wrong columns when batches were combined positionally.

Impact

Repeated Excel writes now preserve all accumulated rows. Columns are unioned in first-seen order and row values remain aligned with their column names, including dynamic dictionary outputs.

Validation

• 9 passed using pytest 9.0.2
• covered Excel append and overwrite accumulation
• covered reordered and dynamic columns
• covered prevention of partial recipe-batch writes
• focused run emitted eight existing pandas SettingWithCopyWarning warnings and no failures

Workflow

This PR targets the dev staging destination for live validation. After staging acceptance, the same recovery/excel-batch-writes branch should be proposed separately to main; dev should not be merged into this branch or into main.