parity(8a): transclusion attr quote-stripping

`{{url|alt|key="quoted value"}}` syntax was already parsed end-to-end
through to `TransclusionNode::attrs`, but the value retained the
surrounding `"…"` (or `'…'`) characters verbatim. The HTML renderer
then escaped them into `"`, producing `style=""border: 1px""`
instead of clean `style="border: 1px"`.

Extracted a small `insert_attr` helper that splits on `=`, trims, and
strips matching single- or double-quote pairs from the value before
inserting. Unquoted values pass through unchanged.

Tests: 4 new in crates/nuwiki-core/tests/transclusion_attrs.rs
covering double-quote stripping, single-quote stripping, unquoted
pass-through, and the clean-HTML rendering shape (no `"`).

Note: this commit covers the "transclusion attrs" half of the
original Cluster 8 audit. The "multi-line list-item continuation
paragraphs" half requires lexer + parser changes (continuation lines
indented past the marker should attach to the prior list item
instead of becoming a sibling paragraph) and is deferred.

Gates: 429 Rust / 39 Neovim / 12 Vim.

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

crates/nuwiki-core/tests/transclusion_attrs.rs

@@ -0,0 +1,60 @@
+//! Cluster 8 — transclusion attribute parsing. The basic
+//! `{{url|alt|key=val}}` shape was already wired; this test pins
+//! down quote-stripping on quoted values (`key="quoted value"`)
+//! so an HTML renderer doesn't double-emit `"` artefacts.
+
+use nuwiki_core::ast::{BlockNode, InlineNode};
+use nuwiki_core::render::{HtmlRenderer, Renderer};
+use nuwiki_core::syntax::vimwiki::VimwikiSyntax;
+use nuwiki_core::syntax::SyntaxPlugin;
+
+fn parse(src: &str) -> nuwiki_core::ast::DocumentNode {
+    VimwikiSyntax::new().parse(src)
+}
+
+fn first_transclusion(doc: &nuwiki_core::ast::DocumentNode) -> &nuwiki_core::ast::TransclusionNode {
+    let para = match &doc.children[0] {
+        BlockNode::Paragraph(p) => p,
+        _ => panic!("expected paragraph"),
+    };
+    for child in &para.children {
+        if let InlineNode::Transclusion(t) = child {
+            return t;
+        }
+    }
+    panic!("no transclusion in paragraph");
+}
+
+#[test]
+fn transclusion_attrs_strip_double_quotes() {
+    let doc = parse(r#"{{cat.png|cat|style="border: 1px"}}"#);
+    let t = first_transclusion(&doc);
+    assert_eq!(t.url, "cat.png");
+    assert_eq!(t.alt.as_deref(), Some("cat"));
+    assert_eq!(
+        t.attrs.get("style").map(String::as_str),
+        Some("border: 1px")
+    );
+}
+
+#[test]
+fn transclusion_attrs_strip_single_quotes() {
+    let doc = parse("{{cat.png|cat|class='thumb'}}");
+    let t = first_transclusion(&doc);
+    assert_eq!(t.attrs.get("class").map(String::as_str), Some("thumb"));
+}
+
+#[test]
+fn transclusion_attrs_leave_unquoted_values_alone() {
+    let doc = parse("{{cat.png|cat|width=200}}");
+    let t = first_transclusion(&doc);
+    assert_eq!(t.attrs.get("width").map(String::as_str), Some("200"));
+}
+
+#[test]
+fn transclusion_renders_clean_html_with_quoted_attr() {
+    let doc = parse(r#"{{cat.png|cat|style="border: 1px"}}"#);
+    let out = HtmlRenderer::new().render_to_string(&doc).unwrap();
+    assert!(out.contains(r#"style="border: 1px""#), "got: {out}");
+    assert!(!out.contains("""));
+}