[Umbrella] Engineering hygiene sweep — Util layout + tech debt cleanup

[Xinze-Li-Moqian]

Goal

Bundle of four engineering-hygiene tasks targeting OpenGA's Util/ infrastructure and known tech debt, executable independently of and in parallel with the main `explicit-g cascade` refactor (#9). Suitable for a single contributor to take end-to-end.

Sub-tasks (recommended order)

• Reorganize Riemannian/Util/ into themed sub-directories #5 — Reorganize `Riemannian/Util/` into themed sub-directories
• Merge small Util files: OperatorSimp + CovDeriv consolidation #6 — Merge small Util files (`OperatorSimp.lean` consolidation + `CovDeriv` absorption)
• Rename ChartJacobianSmooth / ChartJacobianSmoothness for clarity #7 — Rename `ChartJacobianSmooth` / `ChartJacobianSmoothness` for clarity
• Eliminate set_option backward.isDefEq.respectTransparency false workarounds #8 — Eliminate `set_option backward.isDefEq.respectTransparency false` workarounds

Dependency notes

• Reorganize Riemannian/Util/ into themed sub-directories #5 should land first; Merge small Util files: OperatorSimp + CovDeriv consolidation #6 and Rename ChartJacobianSmooth / ChartJacobianSmoothness for clarity #7 are layered on top (Merge small Util files: OperatorSimp + CovDeriv consolidation #6 happens inside the new `Util/Simp/` and `Util/CovDeriv/` sub-dirs; Rename ChartJacobianSmooth / ChartJacobianSmoothness for clarity #7 renames within `Util/Chart/`).
• Eliminate set_option backward.isDefEq.respectTransparency false workarounds #8 is fully independent and can be done anytime (interleaved or last).
• All four are orthogonal to the explicit-g cascade refactor ([Umbrella] Explicit-g cascade: drop [HasMetric I M] auto-resolution from Riemannian stack #9) which is the main-line work. No file-level conflicts as long as Reorganize Riemannian/Util/ into themed sub-directories #5 lands before [Umbrella] Explicit-g cascade: drop [HasMetric I M] auto-resolution from Riemannian stack #9 starts, or [Umbrella] Explicit-g cascade: drop [HasMetric I M] auto-resolution from Riemannian stack #9 holds off until this umbrella closes.

Per-task scope (estimates)

#	Topic	LOC	Risk
#5	sub-dir reorg	~200 import cascade	low (mechanical)
#6	small-file merge	~100	low
#7	rename	~50	low
#8	transparency workaround removal	~50-100, scattered	low to medium (per-decl proof tweak)

Total estimate: ~400-500 LOC of changes across ~25 files. Probably 2-4 PRs.

Acceptance for the umbrella

• All four sub-issues closed.
• Build clean, sorry count unchanged, all linters at baselines.
• `Riemannian/Util/README.md` updated to reflect new sub-dir layout.

How to work this

Each sub-issue can be a separate PR for reviewability. Recommend in order:

1. PR for Reorganize Riemannian/Util/ into themed sub-directories #5 (sets the new directory layout).
2. PR for Merge small Util files: OperatorSimp + CovDeriv consolidation #6 (merges inside the new layout).
3. PR for Rename ChartJacobianSmooth / ChartJacobianSmoothness for clarity #7 (renames inside `Util/Chart/`).
4. PR for Eliminate set_option backward.isDefEq.respectTransparency false workarounds #8 (independent, any time).

When the umbrella closes, ping the maintainer so we can kick off #9 against the cleaned-up `Util/` baseline.