Streamline ArraySelector, VariadicEvaluate, and perturbation seeding

- In ArraySelector, collapse the two absl::FixedArray specializations
  into a single specialization, replace release-mode CHECK_EQ with
  DCHECK, use absl::FixedArray<T, 0> instead of std::vector<T> for the
  heap fallback, and add a fill-value constructor so residual Jet arrays
  are initialized to kImpossibleValue in a single pass.
- Simplify VariadicEvaluateImpl to expand functor(input[Is]..., output)
  directly via std::index_sequence, removing the intermediate
  std::array copy and std::apply/std::invoke wrappers, and mark hot
  helpers EIGEN_STRONG_INLINE.
- In Make1stOrderPerturbation, rely on dst[i].v being zero-initialized
  on entry by Jet's default constructor (checked with DCHECK) and assign
  dst[i].a and dst[i].v[i + Offset] in-place instead of constructing a
  temporary JetT and re-zeroing v.

Benchmark (Apple M1 Pro, kAutoDiff, kResidualsAndJacobians):
----------------------------------------------------------------------
Benchmark                                     Before    After   Change
----------------------------------------------------------------------
SnavelyReprojectionError                      136 ns  78.4 ns   -42.4%
PhotometricError<8>                           901 ns   748 ns   -17.0%
RelativePoseError                             248 ns   130 ns   -47.6%
Brdf                                          827 ns   255 ns   -69.2%
ColmapOpenCVReprojectionError                 185 ns  61.7 ns   -66.6%
ColmapFullOpenCVReprojectionError             263 ns  86.2 ns   -67.2%
ColmapRigReprojectionError                    391 ns   134 ns   -65.7%
ColmapSampsonError                            100 ns  86.2 ns   -13.8%
ColmapFisheyeReprojectionError                209 ns  84.6 ns   -59.5%
ColmapSphericalReprojectionError             86.1 ns  46.5 ns   -46.0%
LibmvBrownReprojectionError                   225 ns  98.7 ns   -56.1%
LibmvNukeInvertReprojectionError              228 ns  82.8 ns   -63.7%
PoseGraph3dError                              249 ns   139 ns   -44.2%
RosenbrockFirstOrderFunctor<4>               10.8 ns  7.96 ns   -26.3%
RosenbrockFirstOrderFunctor<16>               188 ns   163 ns   -13.3%
RosenbrockFirstOrderFunctor<24>               399 ns   367 ns    -8.0%
----------------------------------------------------------------------

Change-Id: I0d3af3eb31b476a8fc5703bbbd529d3fb292ac98