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>

Delivers the full Template Designer GUI Foundation epic (canvas dimensions,
text placement, properties panel, save/reopen XML templates) plus the
overdue high-volume render benchmark spike, all verified against the actual
built .exe per Sprint 1's DoD edit. The benchmark found the render path
misses the 100k-record/10-minute target by 5x-10x+ (code/BENCHMARK.md),
logged as a new high-priority backlog story and technical debt entry for
Sprint 3. Sprint Review confirmed all 5 stories against acceptance criteria;
Retrospective confirmed full follow-through on all four Sprint 1 actions
with no new kit-level changes needed.

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