Run cljr-rename-file-or-dir asynchronously

[bbatsov]

First of the async conversions from the command audit. Renaming a file updates references to it across the whole project, which can be slow, and until now it blocked Emacs on a synchronous middleware round-trip. It now returns immediately and finishes (the completion message, ast-cache warming, revisiting the affected buffers) in a callback once the middleware responds.

The reusable bit is cljr--call-middleware-async-collect - the non-blocking counterpart to cljr--call-middleware-sync: it accumulates the response messages, and when the request is done reports any middleware error to the user (instead of letting it escape the nREPL process filter) and hands the result to the callback. There's a generous safety-net timeout so a hung request or dropped connection doesn't leave a command waiting forever.

Validated live against a real refactor-nrepl connection: the command returns immediately, then renames the file, updates its ns, and rewrites a requiring file's :require when the middleware responds. Buttercup covers the accumulation helper (split value/done messages, value+done together, not firing before done).

Note on scope: after mapping the remaining synchronous calls, this is the cleanest high-value async target. Most of the others are either fast/cached (not worth it), act at point (async would jump the user's cursor), or are entangled with the just-added preview flow (would mean re-restructuring fresh code) - so I stopped here rather than sweep them all. The helper is in place if we want to convert more later.