Default-constructed FLINT DMat specializations are uninitialized and corrupt the heap on destruction

Reported by Claude Opus 5, running as Claude Code on behalf of @d-torrance. The analysis and reproduction above are AI-generated; the abort was observed in a local build, but please treat the suggested fix as unverified.

Summary

The five FLINT-backed DMat specializations have a default constructor that
never initializes the underlying flint matrix, while their destructor clears it
unconditionally. Default-constructing one of these is undefined behavior:
numRows() and numColumns() read uninitialized memory, and destruction
corrupts the heap. The two GF specializations additionally dereference the
null coefficient-ring pointer, because their destructor reaches through
ring() to obtain the flint context.

The generic DMat template is not affected — its default constructor sets
mNumRows, mNumColumns and mArray, and its destructor is guarded by the
length being zero.

Affected code

Each is the same shape. From dmat-zzp-flint.hpp:

  DMat() : mRing(0) {}                          // line 34: mArray untouched
  ...
  ~DMat() { nmod_mat_clear(mArray); }           // line 45: cleared regardless

and the GF variants also read the null ring while destructing:

  ~DMat() { fq_zech_mat_clear(mArray, ring().flintContext()); }

Compare the generic template, dmat.hpp:31:

  DMat() : mRing(nullptr), mNumRows(0), mNumColumns(0), mArray(nullptr) {}

Reproducing

DMat<M2::ARingZZpFlint> empty;
std::cout << empty.numRows() << "\n";   // uninitialized
```                                     // destructor corrupts the heap

Observed while adding `unit-tests/DMatTest.cpp`, whose typed suite covers all
sixteen `DMat` coefficient rings. The first run reported implausible dimensions
and then aborted:

Expected equality of these values:
empty.numRows()
Which is: 140727511877712
0
double free or corruption (out)

### Expected

A default-constructed `DMat` should be a valid empty matrix for every
coefficient ring, as it is for the generic template: zero rows, zero columns,
and safe to destroy. Fixing it presumably means initializing the flint matrix
to 0x0 in the default constructor, and — for the `GF` specializations — either
storing the flint context or guarding the destructor on a null `mRing`.

### Current test status

`unit-tests/DMatTest.cpp` has `DISABLED_defaultConstruction`, which asserts the
intended behavior and is skipped for the five affected rings. Fixing this
should let it be re-enabled as-is.

Start with the default constructors and destructors in dmat-zz-flint.hpp, dmat-qq-flint.hpp, dmat-zzp-flint.hpp, dmat-gf-flint.hpp, and dmat-gf-flint-big.hpp, comparing them with dmat.hpp. Re-enable DISABLED_defaultConstruction in unit-tests/DMatTest.cpp and run the DMat tests; done means every specialization reports zero dimensions and can be safely destroyed.