Ditch Swift differentiation implementation

[jeanbarrossilva]

Compiling the project with a snapshot version of a Swift 6 toolchain has not been an experience I would blatantly call "delightful".

Sudden SourceKitService crashes

The _Differentiation module also seems unready for non-experimental usage, seen that even trivialities like extending one of its types (e.g., making a type differentiable) may (or may not!) crash SourceKitService. As explained in #40, which introduces fields into Deus and, therefore, requires differentiation:

https://github.com/project-deus/Deus/blob/59455693eb99556ff3b5eb8200e481339962b972/QuantumMechanics/Field/Space.swift#L34-L38

Possible SPM–toolchain incompatibility

On top of that, there seems to be an issue with SPM on such a version of the Swift toolchain: setting it as that with which xcbuild builds and tests the project errors with:

xcodebuild: error: Could not resolve package dependencies:
  fatalError
  <unknown>:0: warning: legacy driver is now deprecated; consider avoiding specifying '-disallow-use-new-driver'
<unknown>:0: error: unable to execute command: <unknown>

fatalError

(Which is rather funny: why is fatalError being output? Why is the warning being run as a shell command? 🥸)
(See #44 and https://github.com/project-deus/Deus/actions/runs/17702698250/job/50310350535.)

---

Because of these issues, which are halting advancements on the project, insisting on using differentiation implemented by Swift has proven to not be a good idea. The solution appears to be to do so manually.