SMat equality ignores sparse row positions and unequal list lengths

Nobody has claimed this yet.

`SMat::is_equal` can return true for matrices with different sparse supports. In `Macaulay2/e/basic-mutable-matrices/smat.hpp`, `vec_equals` compares coefficients but does not compare row indices or check that both lists end together.

The following direct engine C++ examples use `SMat` over ZZ/101. Each pair has shape 3 x 1 and must compare unequal:

| Left column (top to bottom) | Right column | Actual `is_equal` |
| --- | --- | --- |
| `[0, 0, 0]` | `[2, 0, 0]` | `true` |
| `[2, 0, 0]` | `[0, 2, 0]` | `true` |
| `[0, 2, 0]` | `[3, 2, 0]` | `true` |

Minimal C++ reproduction (inside the engine unit-test harness):

```cpp
M2::ARingZZp ring(101);
SMat left(ring, 3, 1), right(ring, 3, 1);
M2::ARingZZp::Element two(ring);
ring.set(two, 2);
left.set_entry(0, 0, two);
right.set_entry(1, 0, two);
EXPECT_FALSE(left.is_equal(right)); // fails: actual is true
```

All three cases were reproduced with a Debug CMake engine build on macOS arm64. The production header was unchanged. Equality should compare row positions, coefficients, and the lengths of the sparse lists. This was found while adding dedicated SMat engine unit tests.

Start in Macaulay2/e/basic-mutable-matrices/smat.hpp, at SMat::is_equal and its vec_equals helper. Run the engine unit-test harness with the provided ZZ/101 sparse-matrix examples, then verify that equality accounts for row positions, coefficients, and matching sparse-list lengths.

Written by the indexing model from the issue text.