Avoid invalidated column-cell references in LP pivot paths (#9783)

MSVC ASan reports showed a container-overflow in LP tableau pivoting,
reproducible from both examples and solver tests (issue #9781). The
failure came from reading a `column_cell` through a reference after
pivoting removed that entry from the backing column.

- **Root cause**
- `pivot_column_tableau` and the analogous Diophantine elimination loop
both held `auto& c = column.back()` across a call
(`pivot_row_to_row_given_cell`) that immediately removes that very cell
from the column via `remove_element`.
- After the mutation, the subsequent read `c.var()` used for bookkeeping
observed invalid memory.

- **Change**
- Record the affected row in the bookkeeping set (`m_touched_rows` /
`m_changed_rows`) by reading `c.var()` **before** the pivot call, while
the back cell is still valid.
- Make `static_matrix::pivot_row_to_row_given_cell` return `void`
instead of `bool`. Its result (`!rowii.empty()`) was always `true`: both
callers keep the matrix at full row rank (the tableau basis columns form
an identity submatrix; the Diophantine `m_l_matrix` stays invertible),
so an elementary row operation can never empty a row. The dead `if
(!...) return false;` early-exit in `pivot_column_tableau` is removed
and replaced with a `SASSERT(!rowii.empty())` documenting the invariant.

- **Affected code paths**
- `src/math/lp/static_matrix.h`, `src/math/lp/static_matrix.cpp`,
`src/math/lp/static_matrix_def.h`
  - `src/math/lp/lp_core_solver_base_def.h`
  - `src/math/lp/dioph_eq.cpp`

- **Behavioral impact**
  - No algorithmic change to pivoting.
- Removes the stale-reference hazard in the loops that repeatedly
eliminate entries from a column.

```c++
while (column.size() > 1) {
    auto& c = column.back();
    SASSERT(c.var() != piv_row_index);
    if (m_touched_rows != nullptr)
        m_touched_rows->insert(c.var());
    m_A.pivot_row_to_row_given_cell(piv_row_index, c, j);
}
```

- **Verification**
- Reproduced the exact issue #9781 failure on a local ASan build
(`container-overflow` in `pivot_column_tableau`) using the pre-fix code,
and confirmed it is gone with this change.
- The 4 reported tests pass clean under ASan: `c_example`,
`cpp_example`, `test-z3 get_implied_equalities`, `test-z3 quant_solve`.
  - Full `test-z3 /a` suite: 89 passed, 0 failed, 0 ASan errors.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nikolaj Bjorner <nbjorner@microsoft.com>
Co-authored-by: Lev Nachmanson <levnach@hotmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/math/lp/dioph_eq.cpp

@@ -709,8 +709,8 @@ namespace lp {
             while (column.size() > 1) {
                 auto& c = column.back();
                 SASSERT(c.var() != last_row_index);
-                m_l_matrix.pivot_row_to_row_given_cell(last_row_index, c, j);
                 m_changed_rows.insert(c.var());
+                m_l_matrix.pivot_row_to_row_given_cell(last_row_index, c, j);
             }
         }