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/lexer.rs

@@ -68,8 +68,9 @@ pub enum VimwikiTokenKind {
     // ----- Tables -----
     /// `|` cell separator.
     TableSep,
-    /// A header-separator row, e.g. `|---|---|`.
-    TableHeaderRow,
+    /// A header-separator row, e.g. `|---|---|`. Carries per-cell
+    /// Markdown-style alignment markers (`:--`, `--:`, `:--:`).
+    TableHeaderRow(Vec<crate::ast::TableAlign>),
     /// A cell whose only content is `>`, meaning "merge with cell to the left".
     TableColSpan,
     /// A cell whose only content is `\/`, meaning "merge with cell above".
@@ -563,14 +564,20 @@ impl<'src> LexState<'src> {
             return false;
         }
 
-        // Header separator row: every cell content is dashes (e.g. "|---|---|").
+        // Header separator row: every cell is `-`s (with optional `:`
+        // anchors at one or both ends for alignment) or whitespace.
         if trimmed
             .split('|')
             .filter(|s| !s.is_empty())
-            .all(|cell| cell.chars().all(|c| c == '-' || c == ' ' || c == '\t'))
+            .all(is_sep_cell)
         {
+            let aligns: Vec<crate::ast::TableAlign> = trimmed
+                .split('|')
+                .filter(|s| !s.is_empty())
+                .map(parse_sep_alignment)
+                .collect();
             let span = Span::new(self.line_start_pos(), self.line_end_pos(line));
-            self.push(VimwikiTokenKind::TableHeaderRow, span);
+            self.push(VimwikiTokenKind::TableHeaderRow(aligns), span);
             self.emit_newline(line);
             return true;
         }
@@ -1199,6 +1206,29 @@ fn looks_like_list_marker(s: &str) -> bool {
     list_marker_at(s).is_some_and(|(_, len)| s.as_bytes().get(len) == Some(&b' '))
 }
 
+/// Is this cell content a valid separator-cell body? Allows leading /
+/// trailing `:` for alignment and inner runs of `-` plus whitespace.
+fn is_sep_cell(cell: &str) -> bool {
+    let t = cell.trim();
+    if t.is_empty() {
+        return false;
+    }
+    let body = t.trim_start_matches(':').trim_end_matches(':');
+    !body.is_empty() && body.chars().all(|c| c == '-')
+}
+
+fn parse_sep_alignment(cell: &str) -> crate::ast::TableAlign {
+    let t = cell.trim();
+    let l = t.starts_with(':');
+    let r = t.ends_with(':');
+    match (l, r) {
+        (true, true) => crate::ast::TableAlign::Center,
+        (true, false) => crate::ast::TableAlign::Left,
+        (false, true) => crate::ast::TableAlign::Right,
+        _ => crate::ast::TableAlign::Default,
+    }
+}
+
 fn checkbox_at(s: &str) -> Option<(CheckboxState, usize)> {
     let b = s.as_bytes();
     if b.len() < 3 || b[0] != b'[' || b[2] != b']' {