Matrix % Matrix requires free modules while quotientRemainder does not

Nobody has claimed this yet.

This issue was triaged from [`bugs/dan/0-quotient-for-non-free-modules`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/0-quotient-for-non-free-modules), one of the 857 files removed from the pre-GitHub `bugs/` tree by [`d2c8d27826`](https://github.com/Macaulay2/M2/commit/d2c8d27826) and catalogued in [#36](https://github.com/Macaulay2/M2/issues/36). **The commentary below was written by Claude (Claude Opus 5, via Claude Code)**, not by @d-torrance, whose account posted it -- please weigh it accordingly.

### The original file, verbatim

```text
For quotientRemainder(Matrix,Matrix) and quotient(Matrix,Matrix), if the
source of the first map is not free, the resulting quotient may not be well-defined.

Think about the meaning of this, mathematically, and decide what to do.

Currently Matrix % Matrix requires free modules, inconsistently with quotientRemainder.
```

### Where it stands today

The inconsistency the file reports still stands, and it is easy to see side by side:

```m2
i1 : R = QQ[x,y]; M = R^1/(x^2); N = R^1/(x);
i2 : f = map(N, M, matrix{{1_R}}); g = map(N, N, matrix{{1_R}});
i3 : isFreeModule source f
o3 = false

i4 : first quotientRemainder(f, g)
o4 = matrix {{1}} -- accepted

i5 : f % g
error: expected maps from free modules -- Core/matrix2.m2:482
```

So `quotientRemainder` answers for a non-free source and `%` refuses the same input. `g \\ f` errors on
it too.

### The mathematical question the file raises

Dan's own framing is worth keeping: when the source of the first map is not free, the quotient *may not
be well defined*, so the right answer might be to make `quotientRemainder` stricter rather than `%`
more permissive. What is clearly wrong is having the two disagree silently.

### There is an announced fix that has not arrived

**#3222** (merged 2024-05-10) added the deprecation warning for `Matrix \\ Matrix` explicitly *"in
preparation for a PR in the next release which will change `g \\ f` to call `quotient'(f, g)` … and
which will add a new algorithm that works for arbitrary modules rather than only free modules, as well
as maps of coherent sheaves"*, crediting **#1448** as the design discussion with @mikestillman. The
deprecation landed; the algorithm did not.

_{`open` · disposition `issue` · source of truth: [`bug-triage/catalog.tsv`](https://github.com/d-torrance/M2/blob/bug-triage/bug-triage/catalog.tsv)}

Start with Core/matrix2.m2:482 and compare the implementations of quotientRemainder, Matrix % Matrix, and Matrix \\ Matrix using the non-free-module example in the issue. Read the context from #3222 and the design discussion in #1448. Done means resolving the inconsistent behavior with a documented, mathematically justified direction.

Written by the indexing model from the issue text.