Identify and implement top 5 code reduction

Identify top 5 opportunities to massively reduce the amount of code (lines of code) in any part of the repo: - Similar functionality that can be unified and reused - Code that has been valuable but no more - A front end component or a group of them duplicate another component/page functionality. Use the most modern and nice looking - etc. Now implement them :)

Five code-reduction cuts shipped: 164 route stubs folded onto @on_thread (new) and @on_session, duplicate tests parametrised, e2e viewports single-homed, CommentOut base. Net −1,153 lines/143 files. Quick gate, 4,300+ API tests, jaahd, web, Playwright green; 3 review lanes pass. PR #7835 queued to the merge lane. Deferred (owner call): legacy runtime retirement, types.ts generation. No UI surface affected. Memory: used mem-25288/35351/35380; proposed 1 gotcha.