feat(render): list_ignore_newline / text_ignore_newline = false → <br>

Implements the deferred half of these options: a soft line break inside a
paragraph / list item can render as <br /> instead of a space.

- Parser: soft breaks now emit a dedicated InlineNode::SoftBreak node
  (parse_inline_seq K::Newline + the list-continuation synthetic join) instead
  of collapsing to Text(" "). Added SoftBreakNode + the variant.
- All inline consumers treat SoftBreak as whitespace: AST visitor (no-op),
  text extraction (index.rs + lib.rs → space), nav/semantic_tokens span_of_inline,
  semantic emit (skip like Text), parser span_of_inline.
- Renderer: render_inlines_break renders top-level SoftBreaks as the
  context-specific string; render_paragraph uses text_ignore_newline,
  render_list_item uses list_ignore_newline; default render_inline → space.
  with_newline_handling builder; export.rs wires both from HtmlConfig.

Default (true/true = upstream default) behaviour is unchanged (space). Updated
3 core tests + 1 parser test that asserted the old Text(" ") shape to treat
SoftBreak as whitespace. New test render_page_html_ignore_newline_controls_soft_breaks.

Gap doc: P3 Config item closed — only the deferred markdown cluster remains.
Full rust suite + clippy clean; Neovim 307, Vim 301/18/21.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-core/tests/parser.rs

@@ -91,12 +91,13 @@ fn paragraph_spans_multiple_lines() {
     let BlockNode::Paragraph(p) = &doc.children[0] else {
         panic!("expected paragraph");
     };
-    // Contents: Text("Hello"), Text(" ") (from soft newline), Text("world")
+    // Contents: Text("Hello"), SoftBreak (from soft newline), Text("world").
     let text_concat: String = p
         .children
         .iter()
         .filter_map(|n| match n {
-            InlineNode::Text(t) => Some(t.content.as_str()),
+            InlineNode::Text(t) => Some(t.content.clone()),
+            InlineNode::SoftBreak(_) => Some(" ".to_string()),
             _ => None,
         })
         .collect();