parity(7): table cell alignment markers (|:--|--:|:--:|)

The Markdown-style alignment anchors on a table's header-separator
row now propagate from the lexer through the parser into the AST,
and the HTML renderer emits a `style="text-align: …;"` attribute on
each affected `<th>` / `<td>`.

  |:--|   → left
  |--:|   → right
  |:--:|  → center
  |---|   → default (no style attr)

Encoding choice: rather than threading the source string into the
parser, the lexer now extracts per-cell alignment when it recognises
the separator row and carries the `Vec<TableAlign>` inline on the
`TableHeaderRow` token. The parser unpacks it into `TableNode`'s
new `alignments: Vec<TableAlign>` field. Cells past the end of the
vector render with the default alignment.

`TableAlign` lives in `nuwiki_core::ast::block` alongside the table
nodes and is re-exported via `nuwiki_core::ast`.

Tests: 4 new in crates/nuwiki-core/tests/vimwiki_table_alignment.rs
covering plain dashes, all three anchor flavours, the HTML output
shape, and the no-style-attr case. The existing
`vimwiki_lexer::table_header_separator_row` was updated to assert
the new payload shape.

Gates: 425 Rust / 39 Neovim / 12 Vim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-core/src/render/html.rs

@@ -310,7 +310,7 @@ impl HtmlRenderer {
                     in_tbody = true;
                 }
             }
-            self.render_table_row(row, row_idx, &layout, w)?;
+            self.render_table_row(row, row_idx, &layout, &n.alignments, w)?;
         }
         if in_thead {
             w.write_all(b"</thead>\n")?;
@@ -326,6 +326,7 @@ impl HtmlRenderer {
         n: &TableRowNode,
         row_idx: usize,
         layout: &[Vec<CellLayout>],
+        alignments: &[crate::ast::TableAlign],
         w: &mut dyn Write,
     ) -> io::Result<()> {
         w.write_all(b"<tr>")?;
@@ -341,7 +342,11 @@ impl HtmlRenderer {
             match info {
                 CellLayout::Skip => continue,
                 CellLayout::Lead { colspan, rowspan } => {
-                    self.render_table_cell(cell, n.is_header, colspan, rowspan, w)?;
+                    let align = alignments
+                        .get(col_idx)
+                        .copied()
+                        .unwrap_or(crate::ast::TableAlign::Default);
+                    self.render_table_cell(cell, n.is_header, colspan, rowspan, align, w)?;
                 }
             }
         }
@@ -354,6 +359,7 @@ impl HtmlRenderer {
         is_header: bool,
         colspan: u32,
         rowspan: u32,
+        align: crate::ast::TableAlign,
         w: &mut dyn Write,
     ) -> io::Result<()> {
         let tag = if is_header { "th" } else { "td" };
@@ -364,6 +370,15 @@ impl HtmlRenderer {
         if rowspan > 1 {
             write!(w, " rowspan=\"{rowspan}\"")?;
         }
+        let align_attr = match align {
+            crate::ast::TableAlign::Left => Some("left"),
+            crate::ast::TableAlign::Right => Some("right"),
+            crate::ast::TableAlign::Center => Some("center"),
+            crate::ast::TableAlign::Default => None,
+        };
+        if let Some(a) = align_attr {
+            write!(w, " style=\"text-align: {a};\"")?;
+        }
         write!(w, ">")?;
         self.render_inlines(&n.children, w)?;
         write!(w, "</{tag}>")