feat: enable building as py3-none wheel

[donn]

• also build prunefl as manylinux wheels installable by PIP
• drop version that has never been updated for CalVer
• drop Nix derivation in favor of a shell

[greptile-apps]

Greptile Summary

The PR replaces native tarball releases with a custom PEP 517 backend that builds platform wheels, adopts date-based versions, and replaces the packaged Nix derivation with a development shell.

• Adds sdist and binary-wheel generation plus a Python console-script wrapper.
• Consolidates C++ testing, wheel building, GitHub releases, and download-server publication into one workflow.
• Reworks CMake version-source generation and removes the static VERSION file.
• Removes the Nix flake/package and adds shell.nix.

Confidence Score: 3/5

The PR is not safe to merge until the wheel backend parses successfully and passes the selected package version into the binary build.

Every packaging operation imports a backend containing an invalid f-string, and after that is corrected the current CMake invocation still embeds a date rather than the collision-resolved wheel version.

Files Needing Attention: wheel_build/init.py, CMakeLists.txt

Important Files Changed

Filename	Overview
wheel_build/init.py	Adds the custom packaging backend, but an invalid f-string prevents import and version propagation would desynchronize wheel and binary versions.
CMakeLists.txt	Replaces VERSION-file generation with a pre-build date/version generator whose value must be supplied correctly by packaging callers.
.github/workflows/ci.yml	Consolidates testing, wheel builds, releases, and server uploads, but its wheel path currently reaches the broken backend.
pyproject.toml	Selects the new in-tree PEP 517 backend and declares its build dependencies.
misc/make_version_cc.py	Generates the C++ version symbol from an argument or falls back to the current date.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  Event[Push, pull request, or manual run] --> Tests[C++ build and tests]
  Event --> Version[Choose unused CalVer]
  Tests --> Wheels[cibuildwheel]
  Version --> Wheels
  Wheels --> Backend[PEP 517 wheel_build backend]
  Backend --> CMake[Configure and build prunefl]
  CMake --> Package[Platform wheel]
  Package --> Artifacts[Upload artifacts]
  Artifacts --> Release[GitHub release on non-PR runs]
  Release --> Server[Silimate download server]

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