Base-scenario-first phased generation

[mrwadams]

Automated by agent-loop — pi worked this issue on a fresh clone of mrwadams/attackgen; changes were gated outside the agent on agent-1.

Closes #48

How to test

git fetch origin && git checkout agent/issue-48
python3 -m venv .venv && . .venv/bin/activate
pip install -r requirements.txt
streamlit run "00_👋_Welcome.py"

Then open the printed local URL.

Confirm each acceptance criterion:

• The base scenario, base download, and Navigator layer become available as soon as the base stream completes, while an optional narrative is still generating
• The Assistant handoff is set from the base result and does not wait for the optional narrative
• An input snapshot is captured at Generate press; rendering and downloads use the snapshot, not mutable current widgets
• The active phase and elapsed time are visible during generation; duration messaging acknowledges multi-minute runs
• Coordinator tests cover the phase sequence, base-result persistence before optional enrichment, captured-input metadata, and streamed partial-text visibility using a controllable streaming fixture

Gates (non-authoritative)

• ✅ files_non_empty — 5 non-empty file(s) changed
• ✅ containment — diff stays within the allowed lane
• ✅ verify — pytest: 246 passed, no failures (browser/e2e excluded)

Draft. Browser/e2e tests were NOT run in-gate — run them and review before merging. Nothing here is auto-merged.