directSum(List, Function), so that directSum(x, f) means directSum apply(x, f)

This issue was triaged from [`bugs/dan/1-directSum`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/1-directSum), 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
idea from David: make

directSum(x:List,f:Function) be equivalent to directSum apply(x,f)
```

### Where it stands today

Not installed: `directSum(List, Function)` has no method, so David's idea — that `directSum(x, f)` mean
`directSum apply(x, f)` — still has to be written the long way.

### Why file something this small

The intended semantics are stated unambiguously in one line, there is no design question to settle, and
it is self-contained: a method installation plus a documentation node and a test. That combination suits
a first-time contributor, which is why this is on the tracker rather than parked.

### Not a duplicate of the other directSum issues

Several are open and none is about this signature: **#606** (`MutableHashTable`), **#2891** (components
and direct sums of matrices), **#1060** (degrees ignored).

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

Start at the existing directSum methods and the directSum(List, Function) entry point. Add the method installation, documentation node, and test described in the issue. Done means directSum(x, f) has the same behavior as directSum apply(x, f), with the new usage documented and covered by a test.

Written by the indexing model from the issue text.