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/tests/vimwiki_lexer.rs

@@ -352,7 +352,14 @@ fn simple_table_row() {
 
 #[test]
 fn table_header_separator_row() {
-    assert_eq!(lex("|---|---|\n"), vec![TableHeaderRow, Newline]);
+    use nuwiki_core::ast::TableAlign;
+    assert_eq!(
+        lex("|---|---|\n"),
+        vec![
+            TableHeaderRow(vec![TableAlign::Default, TableAlign::Default]),
+            Newline
+        ]
+    );
 }
 
 #[test]