fix(portfolio): preserve carried Notion origin

[saagpatel]

Summary

• resolve the oldest Notion observation through chained carried-forward artifacts
• preserve live and legacy origin semantics without advancing freshness on each generation
• make CL-FRESH-002 fail when a latest artifact advances its declared origin over predecessor history
• add multi-generation and legacy migration regressions

Verification

• uv run ruff check .
• uv run pytest -q: 2858 passed, 2 skipped
• current canonical artifact is correctly rejected: declared origin 2026-07-11T03:17:38Z resolves to 2026-07-10T09:00:10Z

Boundaries

• no canonical regeneration in this PR
• no scheduler activation, digest, bridge, or Notion write