Add new JSON-RPC API calls, with documentation

[digable1]

Short description of changes

Adds two JSON-RPC API calls: jamulusclient/getDirectories and jamulusclient/getCurrentDirectory.

jamulusclient/getDirectories: Gets the current list of directories.
jamulusclient/getCurrentDirectory: Gets the current directory the client is referencing.

Does this change need documentation? What needs to be documented and how?

JSON-RPC.md has been updated to include documentation for these new API calls

Status of this Pull Request

What is missing until this pull request can be merged?

Nothing

Has been smoke tested to validate we're returning what we think we are.

• [x ] I've verified that this Pull Request follows the general code principles
• [ x] I tested my code and it does what I want
• [ x] My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

AUTOBUILD: Please build all targets

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com
(100% done under Claude Pro - which means Anthropic explicitly states that no ownership/control is asserted)

[ann0see]

Please add a note if this was coded with Claude (in this PR)

[digable1]

line re-formatted, should now satisfy clang-format (surprised that C style doesn't like splitting lines to avoid left-right scroll, but ok - so used to Java/Javascript/Typescript styles where that is either encouraged or at least not flagged)

Also added the callout to Claude (as stated in the PR, because I used Claude Pro there is no copyright/ownership issue - Anthropic explicitly calls that out).

[ann0see]

Please push the changes to this PR. It doesn't seem to be pushed.

[digable1]

Take 2 (yeah I'm supposed to know what I'm doing - whoops!).

[pljones]

Is there already a "set current directory" call? Without one, I'm not sure of the purpose here.

Please clarify how this meets the "keep it simple and small" principle.

[digable1]

The purpose is to obtain ping time for that server. This means I do need to know what the current directory is so I can monitor serverInfoReceived and match the address to get it.

I don't need to play puppet to the Jamulus UI by being able to set the current directory (I personally don't have a use case for it, and couldn't image anybody else needing it either). And not considering setters avoids error handling for parameter validation.