enzyme: opt-in Clang/Enzyme build option + AD smoke test

[krystophny]

What

Add an opt-in Clang/Enzyme build path and a toolchain smoke test:

• VMECPP_ENABLE_ENZYME CMake option, off by default. When on, it requires
  a Clang compiler and -DVMECPP_ENZYME_PLUGIN=/path/to/ClangEnzyme-NN.so, and
  builds the enzyme_smoke test target.
• common/enzyme/enzyme.h: thin declarations of the Enzyme autodiff intrinsics
  and the activity markers, with the allocation constraint that shapes every
  differentiable kernel in this stack documented in place.
• common/enzyme/enzyme_smoke_test.cc: differentiates a scalar objective over
  Eigen::Map'd caller buffers and checks reverse- and forward-mode gradients
  against the closed form and central finite differences.

Why

Stacked on #5. Enzyme differentiates LLVM IR through a Clang plugin, so the
differentiable VMEC++ work needs a Clang build and a way to attach the plugin.
This patch adds that switch and a test that fails loudly if the plugin is not
attached or if Enzyme cannot differentiate the Eigen::Map buffer pattern the
later kernels depend on. It adds no production code; with the option off, the
build is byte-for-byte the previous one.

The smoke test also pins down the one hard Enzyme constraint for this codebase:
Enzyme's allocation analysis does not track Eigen's aligned allocator, so a
dynamic-size Eigen heap temporary crossing the differentiated call aborts with
"freeing without malloc". Differentiable kernels therefore operate on
caller-owned buffers via Eigen::Map. The test exercises exactly that pattern.

Verification

Configure and build with Clang 21.1.8 and the ClangEnzyme-21 plugin:

-- Enzyme plugin: .../ClangEnzyme-21.so
cfg=0
build=0 (0 errors)

ctest:

    Start 1: enzyme_smoke
1/1 Test #1: enzyme_smoke .....................   Passed    0.00 sec
100% tests passed, 0 tests failed out of 1

Smoke test output (reverse and forward both exact, agree with finite
differences):

enzyme smoke test (n=8)
  max|reverse - analytic| = 0.000e+00
  max|forward - analytic| = 0.000e+00
  max|reverse - finite-diff| = 2.646e-08
PASS

No regression with the option off. A default configure (GCC, no Enzyme flags)
succeeds and emits no enzyme_smoke target:

cfg=0  enzyme target present? 0