Add a method for (isSurjective, RingMap)

Nobody has claimed this yet.

This issue was triaged from [`bugs/dan/0-isSurjective-RingMap`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/0-isSurjective-RingMap), 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
add a method for (isSurjective, RingMap)
```

### Where it stands today

Still missing. `isSurjective` has two methods and neither takes a ring map:

```m2
i1 : methods isSurjective
o1 = {(isSurjective, Matrix) }
{(isSurjective, SheafMap)}

i2 : R = QQ[x,y]; f = map(R, R, {x, y});
i3 : isSurjective f
stdio:3:1:(3): error: no method for adjacent objects ...
```

So the request is unchanged since the file was written.

### Notes for whoever picks this up

The companion `isInjective` case is worth checking at the same time, since the two are usually
installed together, and `kernel RingMap` already exists (with a hook-based strategy table at
`Core/ringmap.m2:240`) while there is no corresponding image computation to test surjectivity
against. That is presumably why this one never got added: the obvious implementation compares the
image subring with the target, which is `kernel`'s dual and not free.

_{`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 the existing isSurjective methods and Core/ringmap.m2:240, where kernel RingMap uses a hook-based strategy table. Reproduce the RingMap example from the issue and inspect how isInjective and kernel RingMap are installed. Done means a RingMap is handled by isSurjective, with the companion isInjective case assessed and covered by tests.

Written by the indexing model from the issue text.