Optionally read symbol prompts via completing-read

[bbatsov]

CIDER's symbol prompts (cider-doc, cider-find-var, and friends when there's no symbol at point) read through cider-read-from-minibuffer, which uses completion-at-point rather than completing-read. So completing-read UIs like Vertico, Ivy and Helm don't apply there, and you get the old TAB/*Completions* behavior instead of your usual narrowing UI. We couldn't switch to completing-read in the past because it wants the whole candidate list up front and we can't dump every symbol.

A dynamic collection gets us there: cider-completing-read-symbol reads via completing-read over a table that queries the complete op lazily per input, caches per input, annotates candidates with their type/namespace, and only starts querying once the input reaches cider-completion-symbol-prompt-min-length chars (2), so an empty prompt never asks the runtime for every symbol.

It's gated behind cider-use-completing-read-for-symbol, off by default for now. Verified against a live REPL (lazy querying, the min-length guard, namespaced candidates, annotations, per-input caching) and covered by tests. Docs are in the code-completion page, including a note on eldoc-in-the-prompt behavior.

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