Clarify role of constants in preparation

[gdalle]

DI source

• Specify in Constant docstring that the constant's value shouldn't change after same-point preparation has been performed

DI extensions

• Only allow Zygote to accept Constants and not general Contexts

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (ee11b70) to head (b201d33).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #485   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files         107      107           
  Lines        4298     4298           
=======================================
  Hits         4234     4234           
  Misses         64       64           

Flag	Coverage Δ
DI	98.57% <100.00%> (ø)
DIT	98.37% <ø> (ø)

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