doc: refactor gendoc for better reusability This change separates the gathering of commands/topics/etc from the logic of printing their documentation out.

stream: prefer keeping an open file handle to volatile file instead of copy We will still do copy if too many file handle are open. Currently, have less than 10 volatile files in typical usage, so we should be fine. See inline documentation for details.

stream: remove __getitem__ from the VolatileManager It is no longer used and that API will get in the way of the new order.

stream: open volatile file through the manager To do more subtle things, we need more control.

stream: rename TempCopyManager to VolatileManager We are going to be more subtle in our management of volatile file, so use a more semantic name.

ci: only use the macOS runner if manually invoked Right now, we only have a single machine with a single runner, so don't create a bottleneck by default.

ci: add a runner for macos This is running from a venv, created with the packages from the requirements file in the `test-setup-future-work` topic being discussed separately. We can basically reuse the existing `test_c` configuration as-is, but there was one small issue- if the clone from the previous CI run is still present in `/tmp`, the clone for the current run fails. It's not a problem for the current setup because a new container is instantiated, so the easy fix is to simply delete the clone first. (The Windows CI avoids this by not cloning in the first place.) It looks like it takes ~80m to run the whole test suite on the current system.

branching: merge stable into default

tests: use shlex.quote instead of pipes.quote The pipes module got removed in python 3.13. https://bugs.debian.org/1084553

branchmap-v3: make sure we write the cache after detecting pure-topo mode We were properly detecting the pure topo-case but in some case the one disk cache file was never updated with that information.