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 :)

Implemented the five code-reduction seams the repo has: identical vitest/pytest tests folded into tables (titles kept), a shared plan-gated-section test helper, identical Pydantic models and source bodies single-homed. Net −456 lines, 86 files. Every large candidate (Windows platform, console pairs, legacy redirects, doctrine, identity dual-write) is live by ADR or consumer, documented in the report. Quick gate, API/jaahd/web suites and tsc green; security lane pass, code review fixed, verifier pass. PR #7491 queued in the merge lane. No UI surface affected. Memory: used mem-31297, mem-31667; proposed none.