feat(generate): insert links/tags sections at the cursor line too

Extend the cursor-line insertion from :NuwikiTOC to the other buffer
list generators: :NuwikiGenerateLinks and :NuwikiGenerateTagLinks. They
appended at end-of-file; a fresh section now goes at the cursor line
(with a leading blank), matching :NuwikiTOC. An existing section is still
refreshed in place, and the auto_generate_*-on-save hooks are unaffected
(they only ever replace).

Shared section_insert() helper computes the insert position/block for all
three (cursor line clamped to the document, else the per-command fallback:
top of file for TOC, EOF for links/tags). Clients send the 0-based cursor
line; handlers parse it via parse_uri_line_arg and thread it through
links_edit / tag_links_edit. Rebuild variants pass None.

Tests: links_edit_inserts_at_cursor_line + tag_links_edit_inserts_at_cursor_line.
577 passed, clippy clean, keymap harnesses green. Docs updated.

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

doc/nuwiki.txt

@@ -530,7 +530,8 @@ Page generation ~
 
                                                        *:NuwikiGenerateLinks*
 :NuwikiGenerateLinks
-    Insert a flat list of every page in the wiki under the cursor.
+    Insert a flat list of every page in the wiki. A new section is inserted at
+    the cursor line; an existing one is refreshed in place.
 
                                                           *:NuwikiCheckLinks*
 :[range]NuwikiCheckLinks
@@ -556,7 +557,9 @@ Tags ~
                                                     *:NuwikiGenerateTagLinks*
 :NuwikiGenerateTagLinks [tag]
     Insert a section linking every page that has `<tag>`. With no
-    argument, generates a section per tag found in the workspace.
+    argument, generates a section per tag found in the workspace. A new
+    section is inserted at the cursor line; an existing one is refreshed in
+    place.
 
                                                          *:NuwikiRebuildTags*
 :NuwikiRebuildTags[!]