Add random(Sequence, Ring) to Core for generating several random elements at once

This issue was triaged from bugs/dan/0.1-random-ring-elements, one of the 857 files removed from the pre-GitHub bugs/ tree by d2c8d27826 and catalogued in #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

add to Core:

    i38 : random(Sequence,Ring) := o -> (degs,R) -> for d in deepSplice degs list random(d,R);

    i39 : random((4:1,3:2),R)

	   9   5   3      6 2  7 2  5 2
    o39 = {-x, -x, -x, x, -x , -x , -x }
	   2   6   2      7    9    6

    o39 : List

    i40 : random((4:{1},3:2),R)

	      5   1   7   3 2   2  5 2
    o40 = {x, -x, -x, -x, -x , x , -x }
	      4   5   9   8        4

    o40 : List

Where it stands today

Not added: lookup(random, Sequence, Ring) is null, and random((4:1, 3:2), R) errors.

The file supplies the implementation

random(Sequence, Ring) := o -> (degs, R) -> for d in deepSplice degs list random(d, R)

and the deepSplice is what makes the mixed form work, since it flattens degree lists as well as
repetitions:

random((4:1, 3:2), R)        -- four of degree 1, three of degree 2
random((4:{1}, 3:2), R)      -- same, with a multidegree written as a list

Why it belongs in Core

Generating several random elements of given degrees is a two-line idiom every package writes for itself,
and the sequence spelling (4:1, 3:2) reads better than the apply/join it replaces. There is no
mathematical decision to make here — only whether Core wants the method.

Not a duplicate

#877 and #4161 both concern the one-argument random(List); #4161 wants it to mean "a random
element", with shuffle(List) taking over the permuting. This would add a two-argument form alongside
that, so it is worth cross-referencing there rather than merging.

_{open · disposition issue · source of truth: bug-triage/catalog.tsv}

Start in Core at the random(Sequence, Ring) method entry point and inspect lookup(random, Sequence, Ring), which is currently null. Use the two documented examples, random((4:1, 3:2), R) and random((4:{1}, 3:2), R), to verify that both forms generate the expected list of random elements.