docs(epic-3): add story context XML files for all stories

Story Context Generation Complete:
- 3-1: PDF Generation API Route context (8 ACs, Puppeteer interfaces)
- 3-2: PDF Download Composable context (8 ACs, blob handling)
- 3-3: Download Button Integration context (8 ACs, UButton binding)

Each context includes:
- Acceptance criteria mapping
- Relevant code artifacts and snippets
- Interface definitions
- Constraints and dependencies
- Test ideas per AC

Status Updates:
- All 3 stories: drafted → ready-for-dev

Files Created:
- docs/sprint-artifacts/3-1-create-pdf-generation-api-route.context.xml
- docs/sprint-artifacts/3-2-create-pdf-download-composable.context.xml
- docs/sprint-artifacts/3-3-connect-download-button-to-pdf-generation.context.xml

Files Modified:
- docs/sprint-artifacts/sprint-status.yaml
- docs/sprint-artifacts/3-1-create-pdf-generation-api-route.md
- docs/sprint-artifacts/3-2-create-pdf-download-composable.md
- docs/sprint-artifacts/3-3-connect-download-button-to-pdf-generation.md

Ready for: Dev Agent implementation

docs/sprint-artifacts/sprint-status.yaml

@@ -61,7 +61,7 @@ development_status:
   # FRs: FR10-14
   # ═══════════════════════════════════════════════════════════════
   epic-3: contexted
-  3-1-create-pdf-generation-api-route: drafted
-  3-2-create-pdf-download-composable: drafted
-  3-3-connect-download-button-to-pdf-generation: drafted
+  3-1-create-pdf-generation-api-route: ready-for-dev
+  3-2-create-pdf-download-composable: ready-for-dev
+  3-3-connect-download-button-to-pdf-generation: ready-for-dev
   epic-3-retrospective: optional