extra-dev: coq-prosa compatibility shim for rocq-prosa

[JasonGross]

Split out of #3821, which adds the same shim for four other packages. Opened as a
draft because this row is expected to be red, and not because of the shim.

coq-prosa has no .dev row while its twin rocq-prosa does, and both install to
logpath:prosa. So on a dev switch there is nothing at the coq-prosa name that
resolves without overwriting the twin's files. This adds a coq-prosa.dev that
installs no files of its own and only depends on rocq-prosa {= version} — the same
shape as the 14 coq-*/rocq-* pairs already in extra-dev that are wrapped this
way (coq-mathcomp-*, coq-equations, coq-elpi, coq-hierarchy-builder, ...).

Why it is red

The pre-existing rocq-prosa.dev does not build against the dev prover. CI builds a
changed package's dependencies, so adding a dependency on it is enough to surface the
failure. Both legs of #3821 (4.14.2 and 5.3.0) resolved rocq-prosa at dev and died
inside its own sources:

[ERROR] The compilation of rocq-prosa.dev failed at "make -j2".
File "./util/tactics.v", line 125, characters 21-35:
Error: The reference ssreflect.done was not found in the current environment.
Did you mean ssreflect.hide, ssreflect.phant, ssreflect.hideT or ssreflect.Phant?
make[2]: *** [Makefile.coq:868: util/tactics.vo] Error 1

Nothing in this PR touches rocq-prosa.dev; the diff is one new file. The shim will
go green as soon as rocq-prosa builds at dev, at which point this can be undrafted
and merged as-is.

🤖 Generated with Claude Code

https://claude.ai/code/session_01L9BGQT7XUuubV6C619DW4b