feat(toc): insert a fresh TOC at the cursor line

:NuwikiTOC inserted a new table of contents at the top of the file. It now
inserts at the cursor line instead, so you can place the TOC where you want
it (a blank line separates it from preceding text). An existing TOC is
still refreshed in place, and auto_toc-on-save is unaffected.

The Vim/Lua clients send the 0-based cursor line with nuwiki.toc.generate;
the server threads it into toc_edit (clamped to the document) and falls
back to the top of the file when absent. toc_rebuild_edit passes None.

Tests: toc_edit_inserts_at_cursor_line +
toc_edit_cursor_line_clamped_and_existing_replaced_in_place. 575 passed,
clippy clean, keymap harnesses green. Doc updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

doc/nuwiki.txt

@@ -525,7 +525,8 @@ Page generation ~
 
                                                                 *:NuwikiTOC*
 :NuwikiTOC
-    Generate or refresh the table of contents on the current page.
+    Generate or refresh the table of contents on the current page. A new TOC
+    is inserted at the cursor line; an existing one is refreshed in place.
 
                                                        *:NuwikiGenerateLinks*
 :NuwikiGenerateLinks