Tier 1: add README.md to 20 sub-directories currently lacking one

[Xinze-Li-Moqian]

Part of umbrella #48.

Scope

20 sub-directories under OpenGALib/ lack a README. Add a short README (~30-60 lines) covering: layer position, dependency direction, typical use, conventions specific to the sub-dir.

Reference style: existing READMEs in OpenGALib/Riemannian/Util/Metric/, OpenGALib/Riemannian/Util/CovDeriv/, OpenGALib/Riemannian/Util/Tangent/, OpenGALib/Riemannian/Util/Chart/, OpenGALib/Riemannian/Util/Simp/ (introduced by poet77's PR #41).

Directories needing README

• OpenGALib/GeometricMeasureTheory/Variation
• OpenGALib/GeometricMeasureTheory/Isoperimetric
• OpenGALib/Riemannian/TensorBundle
• OpenGALib/Riemannian/TensorBundle/SmoothOrthoFrame
• OpenGALib/Riemannian/Connection
• OpenGALib/Riemannian/Operators
• OpenGALib/Riemannian/Operators/Bochner
• OpenGALib/Riemannian/Util
• OpenGALib/Riemannian/Curvature
• OpenGALib/Riemannian/Manifold
• OpenGALib/Riemannian/TangentBundle
• OpenGALib/Riemannian/Instances
• OpenGALib/Riemannian/Metric
• OpenGALib/Riemannian/Volume
• OpenGALib/Riemannian/Volume/Util
• OpenGALib/Tests/Linter
• OpenGALib/MetricGeometry/Examples
• OpenGALib/Algebraic/BilinearForm
• OpenGALib/Algebraic/Instances
• OpenGALib/Algebraic/Auxiliary

Acceptance

• README present in each listed directory
• Each README follows the existing style template (layer, deps, conventions)
• No code changes (docs-only PR)

Suggested split

1-2 PRs total; or split by top-level area (Riemannian / GMT / Algebraic / Tensor / Tests) if convenient.

Notes

Both maintainers assigned. Whoever has a quiet block of time picks it up; comment to claim before starting to avoid duplicate drafts.