feat(voteballots): add shell integration payload contract (Slice 6/6) (#714)

Implements local shell payload contract for future p.fMALL Vote shell
consumption. This is a LOCAL data contract only - no route activation,
no public launch, no manifest mutation.

Shell payload fields:
- foundup_id, route_namespace, app_mount (readonly from manifest)
- lines, confidence_label, source_trace_id (preserved from QuickAnswer)
- trail_termination_markers, human_review_triggers (preserved)
- display_ready, truncated, warnings (computed)

WSP 97 constraints enforced:
- LOCAL_SHELL_PAYLOAD_ONLY
- NO_PUBLIC_LAUNCH
- NO_ROUTE_ACTIVATION
- NO_MANIFEST_MUTATION
- ANSWER_LINES_PRESERVED
- CONFIDENCE_LABELS_PRESERVED

Vote PoC Chain Lane: 6/6 slices COMPLETE
Tests: 303 passed (241 existing + 62 new)

Worker-Lane: W6
Slice: VOTE_POC_SHELL_INTEGRATION_PHASE1

Co-authored-by: Foundups Agent <dev@foundups.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 3 people