Easy install

[zasexton]

Current situation

• This PR makes svVascularize installable in two modes:
  • Pure‑Python by default via pip install svv.
  • Accelerated via pip install "svv[accel]" using a new companion wheel svv‑accelerated.
• It also removes the unused hnswlib dependency and HNSWTree code path in favor of
  USearchTree.
• Related items to track/discuss:
  • Packaging: offering both pure and accelerated wheels from the same repo.
  • Naming: reserving the “all” extra for future bundles (accelerators + solvers).

Release Notes

• New: Pure‑Python install path by default; compiled accelerators are now optional.
  • pip install svv → pure‑Python fallbacks.
  • pip install "svv[accel]" (or "svv[accelerated]") → installs the companion
    svv‑accelerated wheel; svv auto‑prefers compiled modules.
• New: Companion wheel “svv‑accelerated”
  • Contains compiled C/Cython extensions under the svv_accel.* namespace.
  • svv transparently aliases svv_accel modules to svv.* imports when present.
• CI: Build and publish both wheels (svv and svv‑accelerated) via cibuildwheel.
  • Pure‑Python svv wheel and sdist.
  • Accelerated svv‑accelerated wheels (no sdist).
• Docs: Updated installation guide (docs/install.html) with:
  • Clear pure vs accelerated instructions.
  • Advanced from‑source option.
  • Snippet to verify which backend is active at runtime.
• Packaging cleanup:
  • Removed hnswlib dependency and HNSWTree implementation; use USearchTree consistently.
  • Updated import sites to drop HNSWTree.
• Minor message tweak: local optimization ImportError now recommends pip install svv[accel].
• Migration notes (potentially breaking):
  • If external code directly imports/uses HNSWTree, migrate to USearchTree (compatible API
    in this context).
  • If docs/scripts previously referenced svv[all] for accelerators, switch to svv[accel].
    The “all” extra is now reserved for a future full bundle.

Documentation

• Installation page updated:
  • Added “Choose pure‑Python or accelerated” section with precise commands.
  • Added verification snippet to confirm loaded backend (.so/.pyd vs .py).
• No API reference changes required; accelerators are auto‑preferred when installed.

Testing

• CI builds both pure‑Python and accelerated wheels on Linux/macOS/Windows for CPython 3.9–
  3.12.
• Existing test suite should pass in both modes (pure and accelerated).
• Packaging workflow verifies sdist creation (for svv) and wheel builds for both
  distributions.
• Follow‑up recommended:
  • Ensure coverage of the aliasing behavior (svv → svv_accel) and the pure/accelerated
    import preference in CI.
  • Confirm downstream workflows (e.g., large‑tree builds, GUI stacks) still succeed in
    both modes.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.