feat(repl): session-context toolbar + completion/popup polish

Adds the bottom-toolbar feature from the original plan plus the
last round of UX polish on the prompt_toolkit candidate popup
that we shaved down through testing:

- **Bottom toolbar** shows the running RF version and the cwd at
  all times — orientation info that's useful when juggling several
  venvs / project directories.
- **Esc** closes the candidate popup and reverts any preview text
  that arrow-navigation had pinned, restoring the user's literal
  pre-popup buffer. Bound only when the popup is open
  (`has_completions` filter) so Esc keeps its Alt-chord prefix
  role everywhere else.
- **Enter** on a highlighted candidate accepts it (popup closes,
  preview becomes permanent). For keyword completions Enter also
  cleans up the cell — deletes any leftover text after the cursor
  in the same cell (so a mid-cell completion replaces the whole
  keyword) and appends a cell separator if no argument follows.
- **Tab** is smart: cycles through the popup when one is open,
  inserts a cell separator in argument context, opens the popup
  otherwise.
- **Multi-line history** survives a session restart as one entry
  (newlines are escaped to `\\n` on disk so the file stays
  readline-compatible).
- **Mid-keyword preview** renders cleanly — `${Lo}` cursor before
  `o`, picking `Log To Console` now shows `Log To Console`, not
  `Log To Consoleog` (we trim the forward-cell tail from the
  state's `original_document` before prompt_toolkit applies the
  preview).
- **TEXT_FRAGMENT** in compound variable lookups (`${PREFIX_${X}}`)
  renders with the variable-name colour, not the argument colour,
  so the eye reads the whole concatenated name as one identifier.
- **EOL / EOS** tokens from Robot's tokenizer are dropped from the
  rendered fragments — `\\n` inside a fragment would otherwise show
  as `^J` at the end of every multi-line buffer line.
- **Trailing whitespace** is padded out to the document's actual
  length so the cursor stays in sync with the buffer (was sticking
  at column 0 when the user kept typing spaces).
- **`_completion.py`** is consolidated — `candidates_for()` is now
  a thin projection over `candidates_for_rich()`, ~200 lines of
  duplicated helpers removed (was an artefact of how the rich
  variant was added in the previous commit).
- **`CELL_SEPARATOR`** + **`find_cell_end()`** added to
  `_completion.py` as named constants and a shared helper, used by
  the smart-Tab / Enter-cleanup paths.

Author: d-biehl

docs/03_reference/repl.md

@@ -166,6 +166,17 @@ The completer runs in a background thread (`complete_in_thread=True`), and Robot
 
 History is shared with the readline backend — same plain-text file, so swapping between the two extras (or having neither) doesn't lose arrow-up recall.
 
+#### Session-context bottom toolbar
+
+A status line at the bottom of the prompt shows two pieces of orientation info at a glance:
+
+```
+ RF 7.4 · cwd: ~/projects/my-suite
+```
+
+- **RF version** — which Robot Framework is actually running (handy when juggling several venvs).
+- **Working directory** — where relative paths in `Import Resource`, `Import Library`, and file-based variables resolve from.
+
 #### Documentation hints in the popup
 
 Each candidate in the completion popup now shows a short context string to its right (prompt-toolkit's `display_meta`), so you know *what* a candidate is before picking it: