docs(readme): simplify structure; fold exhaustive detail; add eigen + tensorvia-cpu

Shorter, more scannable index README (292 → 190 lines):
- Two clear package tables (modular C++23 / third-party compat); the X11/XCB
  runtime cluster, the full dependency tree, the per-package smoke breakdown,
  and the Form A/B examples move into <details> folds.
- Surface the per-package `features` knobs inline (gtest main / cjson utils /
  eigen blas).
- Add the two rows that were missing from the index: `eigen` 5.0.1 and
  `tensorvia-cpu` 0.1.1 (the latter shipped in #47 but was never listed).
- "添加新包" now points at the new add-mcpp-index-package skill for the full SOP.

Author: Sunrisepeak