Reduce codebase by implementing top five refactoring

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 largest safe code reductions (−450 net lines, 17 files): tz table from tzdata zone.tab, cron_retention sweep helpers, shared clock.parse_utc/jsonable, useUnsavedLeave folded into useLeaveGuard, antigravity bridge on the core spool. Recon measured the repo already single-homed (0.07% dup). Quick gate, 941 changed-area tests and a live visual check green; blind review passed, its 4 should-fixes landed. PR #6888 merged into develop (7beb8865). Deferred: legacy redirects, identity/git-pointer contractions, vendored yaml/cryptography, Windows/macOS layer. Memory: used mem-25288; proposed 1 qa.