refactor(tests): group test files by feature, drop phase/cluster naming

Restructures the integration test layout so each file is named after
what it covers, not the implementation phase it landed in.

nuwiki-core (renames only):
  vimwiki_lexer            → lexer
  vimwiki_parser           → parser
  vimwiki_tags             → tags
  vimwiki_table_alignment  → table_alignment
  diary_period             → diary
  list_continuation        → lists
  transclusion_attrs       → transclusion
  + table colspan/rowspan tests moved here from parity_cluster_1

nuwiki-lsp (renames + merges + one split):
  cluster_a_list_rewriters       → commands_lists
  cluster_b_table_rewriters      → commands_tables
  cluster_c_link_helpers +
    phase19_followlink_creates   → commands_links
  phase13_rename_commands        → commands_files
  phase17_colorize               → commands_colorize
  phase17_html_export            → html_export
  phase15_link_health            → link_health
  phase18_multi_wiki             → multi_wiki
  phase19_folding                → folding
  nav                            → navigation
  lsp_helpers                    → helpers
  phase16_diary +
    diary_frequency              → diary
  phase11_plumbing +
    parity_cluster_1 (config)    → index_and_config
  tags_index_and_lsp +
    phase17_backfill             → commands_tags
  phase14_edit_commands          → split into
    commands_checkboxes,
    commands_headings,
    commands_tasks

Net: 30 → 28 integration-test files. 456 → 455 tests (the one
removed test was a dummy `paragraph_render_is_unchanged` whose only
purpose was to keep a `ParagraphNode` import alive in
parity_cluster_1; the import is exercised elsewhere now).

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

crates/nuwiki-lsp/tests/commands_headings.rs

@@ -0,0 +1,83 @@
+//! Heading level rewriter — `nuwiki.heading.addLevel` /
+//! `nuwiki.heading.removeLevel`.
+
+use nuwiki_core::syntax::vimwiki::VimwikiSyntax;
+use nuwiki_core::syntax::SyntaxPlugin;
+use nuwiki_lsp::commands::ops;
+use tower_lsp::lsp_types::Url;
+
+fn parse(src: &str) -> nuwiki_core::ast::DocumentNode {
+    VimwikiSyntax::new().parse(src)
+}
+
+fn dummy_uri() -> Url {
+    Url::parse("file:///tmp/note.wiki").unwrap()
+}
+
+fn one_text_edit(edit: tower_lsp::lsp_types::WorkspaceEdit) -> tower_lsp::lsp_types::TextEdit {
+    let changes = edit.changes.expect("expected changes map");
+    let (_, edits) = changes.into_iter().next().expect("at least one uri");
+    assert_eq!(edits.len(), 1, "expected exactly one text edit");
+    edits.into_iter().next().unwrap()
+}
+
+#[test]
+fn add_heading_level_promotes_h2_to_h3() {
+    let src = "== Sub ==\n";
+    let doc = parse(src);
+    let edit =
+        ops::heading_change_level(src, &doc, &dummy_uri(), 0, 0, true, 1).expect("add_level edit");
+    let te = one_text_edit(edit);
+    assert_eq!(te.new_text, "=== Sub ===");
+}
+
+#[test]
+fn add_heading_level_caps_at_6() {
+    let src = "====== Deep ======\n";
+    let doc = parse(src);
+    let edit = ops::heading_change_level(src, &doc, &dummy_uri(), 0, 0, true, 1);
+    // No-op because level 6 is the cap; clamp matched current level.
+    assert!(edit.is_none());
+}
+
+#[test]
+fn remove_heading_level_demotes() {
+    let src = "=== Mid ===\n";
+    let doc = parse(src);
+    let edit = ops::heading_change_level(src, &doc, &dummy_uri(), 0, 0, true, -1)
+        .expect("remove_level edit");
+    let te = one_text_edit(edit);
+    assert_eq!(te.new_text, "== Mid ==");
+}
+
+#[test]
+fn remove_heading_level_at_h1_is_noop() {
+    let src = "= Top =\n";
+    let doc = parse(src);
+    let edit = ops::heading_change_level(src, &doc, &dummy_uri(), 0, 0, true, -1);
+    assert!(edit.is_none());
+}
+
+#[test]
+fn heading_change_returns_none_when_cursor_not_on_heading() {
+    let src = "paragraph\n";
+    let doc = parse(src);
+    let edit = ops::heading_change_level(src, &doc, &dummy_uri(), 0, 0, true, 1);
+    assert!(edit.is_none());
+}
+
+#[test]
+fn rewrite_heading_with_level_keeps_leading_whitespace_outside_span() {
+    // The lexer's heading span starts at the first `=`, not the line's
+    // start, so the leading whitespace that marks a "centered" heading
+    // lives outside the edit range — preserved automatically. The
+    // rewriter's output is just the `=...=` portion.
+    let src = "  = Title =\n";
+    let doc = parse(src);
+    let h = match &doc.children[0] {
+        nuwiki_core::ast::BlockNode::Heading(h) => h,
+        _ => panic!("expected heading"),
+    };
+    let new = ops::rewrite_heading_with_level(src, h.span, 2).unwrap();
+    assert_eq!(new, "== Title ==");
+}