Remove dead code in the DGCP geodesic-curvature engine

Two behavior-preserving removals in find_gcurvature, verified against
test/dgp.jl + test/lorentz.jl (unchanged pass):

- Delete the unreachable `elseif f_curvature == Affine` branch. The
  chain opens with `f_curvature == Convex || f_curvature == Affine`,
  which catches every Affine atom first, so the later Affine block can
  never run; the `|| f_curvature == Affine` on the Concave branch is
  dead for the same reason and is dropped.
- Delete the duplicate `add_gdcprule(diag, SPD, ...)` in spd.jl. It is
  identical to the `add_gdcprule(LinearAlgebra.diag, ...)` registration
  (diag === LinearAlgebra.diag) and, since gdcprules_dict is keyed by
  function, only overwrote it with the same value.

The always-false `sum_log_eigmax` comparison and the (behaviorally
inert) distance manifold-key collision are left untouched and flagged
in the PR for an intent decision.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ChrisRackauckas

src/gdcp/gdcp_rules.jl

@@ -204,7 +204,7 @@ function find_gcurvature(ex)
             else
                 return GUnknownCurvature
             end
-        elseif f_curvature == Concave || f_curvature == Affine
+        elseif f_curvature == Concave
             if all(enumerate(args)) do (i, arg)
                     arg_curv = find_gcurvature(arg)
                     m = f_monotonicity[i]
@@ -220,15 +220,6 @@ function find_gcurvature(ex)
             else
                 return GUnknownCurvature
             end
-        elseif f_curvature == Affine
-            if all(enumerate(args)) do (i, arg)
-                    arg_curv = find_gcurvature(arg)
-                    arg_curv == GLinear
-                end
-                return GLinear
-            else
-                return GUnknownCurvature
-            end
         elseif f_curvature isa GCurvature
             return f_curvature
         else

src/gdcp/spd.jl

@@ -190,8 +190,6 @@ add_gdcprule(log_quad_form, SymmetricPositiveDefinite, Positive, GConvex, GIncre
 
 add_gdcprule(inv, SymmetricPositiveDefinite, Positive, GConvex, GDecreasing)
 
-add_gdcprule(diag, SymmetricPositiveDefinite, Positive, GConvex, GIncreasing)
-
 # Matrix `log` (both `Arr` and `Matrix{Num}` forms) is defined in atoms.jl via
 # `matrix_atom`; the `@register_array_symbolic` form collides with SymbolicUtils'
 # scalar `log` on Symbolics v7 (see the note there).