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/syntax/vimwiki/parser.rs

@@ -210,7 +210,7 @@ impl<'a> ParseState<'a> {
             K::HorizontalRule => self.parse_horizontal_rule(),
             K::ListMarker { .. } => self.parse_list(),
             K::BlockquoteMarker | K::BlockquoteIndent => self.parse_blockquote(),
-            K::TableSep | K::TableHeaderRow => self.parse_table(),
+            K::TableSep | K::TableHeaderRow(_) => self.parse_table(),
             K::PreformattedOpen { .. } => self.parse_preformatted(),
             K::MathBlockOpen { .. } => self.parse_math_block(),
             K::CommentLine(_) => self.parse_single_comment(),
@@ -620,11 +620,15 @@ impl<'a> ParseState<'a> {
         let mut rows: Vec<TableRowNode> = Vec::new();
         let mut next_is_header = false;
         let mut has_header = false;
+        let mut alignments: Vec<crate::ast::TableAlign> = Vec::new();
 
         while let Some(t) = self.peek() {
             match &t.kind {
-                K::TableHeaderRow => {
+                K::TableHeaderRow(aligns) => {
                     has_header = true;
+                    if alignments.is_empty() {
+                        alignments = aligns.clone();
+                    }
                     if let Some(last) = rows.last_mut() {
                         last.is_header = true;
                     }
@@ -638,7 +642,6 @@ impl<'a> ParseState<'a> {
                     span_end = row.span.end;
                     rows.push(row);
                     if next_is_header {
-                        // Header sep applies to the *previous* row, not this one.
                         next_is_header = false;
                     }
                 }
@@ -649,6 +652,7 @@ impl<'a> ParseState<'a> {
             span: Span::new(span_start, span_end),
             rows,
             has_header,
+            alignments,
         })
     }
 
@@ -870,7 +874,7 @@ fn starts_new_block(kind: &K) -> bool {
             | K::BlockquoteMarker
             | K::BlockquoteIndent
             | K::TableSep
-            | K::TableHeaderRow
+            | K::TableHeaderRow(_)
             | K::PreformattedOpen { .. }
             | K::MathBlockOpen { .. }
             | K::CommentLine(_)