Phase 2: remove the retired crates (final releases, workspace removal, facade slimming, CI simplification)

[shinaoka]

Part of #199 (Phase 2). Blocked by the tenferro absorption (tenferro must no longer depend on strided-einsum2).

• Publish final versions of the four crates carrying the deprecation notice (no yank).
• Remove strided-einsum2, strided-opteinsum, mdarray-opteinsum, ndarray-opteinsum from the workspace on main.
• Slim the facade: drop the einsum2 / opteinsum / mdarray / ndarray modules, the flat einsum re-exports (einsum, einsum_into, einsum_with_pool, einsum_into_with_pool, BufferPool, EinsumCode, EinsumError, EinsumNode, EinsumOperand, EinsumScalar, StridedData, TypedTensor), and the faer / blas / blas-inject feature plumbing from strided-rs/src/lib.rs and strided-rs/Cargo.toml. Bump the version.
• Simplify CI: the faer / blas lanes leave with the crates; keep the retained-crate matrix from the CI issue.
• Final sweep of README / docs references to the removed crates (all file:line references are pinned to origin/main at 9da9b9f.)

[shinaoka]

The tenferro dependency blocker is cleared: tensor4all/tenferro-rs#1591 is closed with evidence at tensor4all/tenferro-rs#1591 (comment), and PR tensor4all/tenferro-rs#1553 removed the stale Cargo edge.

This issue should remain open for its remaining crate deletion, release, and repository cleanup steps; this update does not claim those steps are complete.

[shinaoka]

2026-08-10 棚卸し

obsolete — close as not planned。crate removal/facade slimming は #226 の全9 crate maintenance policy と逆方向。release/CI 改善は #215 等で扱い、本 removal issue には PR を作らず close する。

Umbrella: #230。実装 PR の base は umbrella/issue-burndown-2026-08。