fix(metadata): restore TlsGroup tensor initialization

[CV-GPhL]

Restores NAN initialization of TlsGroup::T, ::L, and ::S that was removed during the Doxygen documentation PR series (#414).

The bug

The original code initialized these tensors to NAN:

SMat33<double> T = {NAN, NAN, NAN, NAN, NAN, NAN};
SMat33<double> L = {NAN, NAN, NAN, NAN, NAN, NAN};
Mat33 S = Mat33{NAN};

After #414 these became:

SMat33<double> T;
SMat33<double> L;
Mat33 S;

Why it matters

The mmCIF writer uses number_or_qmark in src/to_mmcif.cpp:

inline std::string number_or_qmark(double d) {
  return std::isnan(d) ? "?" : to_str(d);
}

With the original NAN initializers, a TlsGroup that was instantiated but never populated would emit ? (unset) for all tensor components in mmCIF output. Without the initializers:

• SMat33<double> T, L hold undefined values → garbage numbers in output
• Mat33 S defaults to identity matrix → emitted as 1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0, indistinguishable from real TLS S-tensor data

A partially populated TlsGroup (e.g. origin and selections set, tensors not yet computed) would silently write misleading numerical data to disk that downstream refinement programs would treat as valid.

Relation to earlier fix

Commit 2c1b0c5 restored the analogous initialization for RefinementInfo::aniso_b but missed the TlsGroup tensors. This PR completes that fix.

Companion PR

Related: #427 fixes a separate missed initialization (GridMeta::spacegroup) from the same PR series.