Completes the CSV Integration and Field Mapping epic by collapsing blank
optional address lines (with a visual distinction between blank data and a
mapping error), and delivers text/field rotation end-to-end: a free-form
angle property that rotates about the element's bounding-box center
(verified against the real Debenu DLL with a pixel-centroid regression
test), plus an interactive drag-to-rotate canvas handle. Closes out Sprint 4
with review and retrospective notes, including resolving the prior
retrospective's product-owner review-independence watch item. Also captures
requirements (not yet sized) for a new composite Address Control epic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Mitigates the high-volume render throughput regression via Debenu document
batching + merge (100k-record benchmark: 315s, ~47.5% under the 10-minute
target), delivers the first CSV field-mapping slice (load headers, bind and
rebind dynamic fields), and closes out Sprint 3 with review and retrospective
notes. Also refines and sizes two new backlog stories for text/field element
rotation, requested mid-sprint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>