docs(lethe-web-ui-aggregates): record execute, verify, review

Mark task Reviewed. Conclusion captures the eight active
deviations, the four Important review findings (all
resolved in 3fbbfc8 and 1818fac), the verify checklist
(positive / negative / invariant / interface checks all
passing), and the deferred items needing user smoke
through the new SPA routes in a real browser.

docs(lethe-web-ui-aggregates): add implementation plan

docs: add roadmap TODO and lethe-web-ui-aggregates design

TODO.md indexes all 9 task specs (4 done/in-progress, 5 deferred or
blocked) plus the carry-overs from the foundation Conclusion. New task
file lethe-web-ui-aggregates covers the backend /projects + /stats
endpoints and the three frontend screens (Projects index, Project
detail, Stats), replacing three of the foundation's stub routes.