Adds context summary

[edgararuiz]

1. Fixed the progress bar — switched from byte-size tracking to field-count,
   then back to size-based with progress_bar_update_done(obj), split the single
   progress_bar_update() into progress_bar_update_text() and
   progress_bar_update_done() for clarity, and simplified the total size
   calculation to object.size(lst) minus examples and usage.

2. Cleaned up rd_translate — moved it to the top of the file, removed the
   redundant rd_trans_size() function, consolidated progress_bar_update_done()
   calls to pass whole nodes instead of subcomponents, and improved all progress
   label wording (dashes instead of colons, consistent plurals, "Creating Context"
   instead of "Context").

3. Extracted rd_find() — pulled the Rd content lookup logic out of
   lang_help() into its own internal function, returning a list with topic,
   package, and content, making it independently callable for debugging.

4. Improved the README — added an explanation of the context feature and why
   it exists, added an ellmer example to the auto-initialize section, fixed all
   double spaces, duplicate badge, comma after "Llama3.2", "your your", moved the
   section-names caveat, and renamed "Debug" to "Debugging".

5. Improved help documentation — fixed grammar and typos across lang-use.R,
   lang-help.R, and help-shims.R, and rewrote several @param descriptions
   to be clearer (.cache, .context_size, backend, topic, and the shims
   description).