phase 2: syntax plugin interface and registry

Add the Lexer/Parser/SyntaxPlugin traits and the SyntaxRegistry per
SPEC.md §6.3.

- `Lexer<Token>` and `Parser<Token>` are typed building blocks: each
  syntax owns its own token alphabet (vimwiki today, markdown later).
- `TokenStream<T>` is a `Vec<T>` newtype so the eager-vs-lazy choice
  (§6.6) can flip later without breaking callers.
- `SyntaxPlugin` is type-erased: it exposes id / display_name /
  file_extensions / parse(text) so the registry can hold heterogeneous
  plugins behind a single trait object. Implementations chain their
  Lexer + Parser inside parse().
- `SyntaxPlugin: Send + Sync` so the LSP server can stash plugins in
  an Arc and share them across handler tasks.
- `SyntaxRegistry` stores `Arc<dyn SyntaxPlugin>`, supports lookup by
  id and by file extension, iteration in registration order.

Tests cover TokenStream round-trip, full lex→parse chain through a
mock plugin, registry lookup hits and misses, and a compile-time
assert_send_sync::<SyntaxRegistry>().

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

crates/nuwiki-core/tests/syntax.rs

@@ -0,0 +1,144 @@
+//! Integration tests for the syntax plugin interface.
+
+use nuwiki_core::ast::{
+    inline::InlineNode, BlockNode, DocumentNode, ParagraphNode, Span, TextNode,
+};
+use nuwiki_core::syntax::{Lexer, Parser, SyntaxPlugin, SyntaxRegistry, TokenStream};
+
+// --- A minimal mock plugin: tokenises the input on whitespace, then wraps
+// the joined text in a single Paragraph. Just enough surface to exercise
+// every trait + the registry.
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct MockToken(String);
+
+struct MockLexer;
+impl Lexer for MockLexer {
+    type Token = MockToken;
+
+    fn lex(&self, text: &str) -> TokenStream<MockToken> {
+        TokenStream::from_vec(
+            text.split_whitespace()
+                .map(|w| MockToken(w.to_owned()))
+                .collect(),
+        )
+    }
+}
+
+struct MockParser;
+impl Parser for MockParser {
+    type Token = MockToken;
+
+    fn parse(&self, tokens: TokenStream<MockToken>) -> DocumentNode {
+        let joined = tokens
+            .iter()
+            .map(|t| t.0.as_str())
+            .collect::<Vec<_>>()
+            .join(" ");
+        let paragraph = BlockNode::Paragraph(ParagraphNode {
+            span: Span::default(),
+            children: vec![InlineNode::Text(TextNode {
+                span: Span::default(),
+                content: joined,
+            })],
+        });
+        DocumentNode {
+            span: Span::default(),
+            children: vec![paragraph],
+            metadata: Default::default(),
+        }
+    }
+}
+
+struct MockPlugin;
+impl SyntaxPlugin for MockPlugin {
+    fn id(&self) -> &str {
+        "mock"
+    }
+    fn display_name(&self) -> &str {
+        "Mock Syntax"
+    }
+    fn file_extensions(&self) -> &[&str] {
+        &[".mock", ".mk"]
+    }
+    fn parse(&self, text: &str) -> DocumentNode {
+        MockParser.parse(MockLexer.lex(text))
+    }
+}
+
+#[test]
+fn token_stream_roundtrip() {
+    let mut s = TokenStream::<u32>::new();
+    assert!(s.is_empty());
+    s.push(1);
+    s.push(2);
+    s.push(3);
+    assert_eq!(s.len(), 3);
+    assert_eq!(s.as_slice(), &[1, 2, 3]);
+    assert_eq!(s.iter().sum::<u32>(), 6);
+    assert_eq!((&s).into_iter().sum::<u32>(), 6);
+    assert_eq!(s.into_vec(), vec![1, 2, 3]);
+
+    let from_vec: TokenStream<&str> = vec!["a", "b"].into();
+    assert_eq!(from_vec.len(), 2);
+    let collected: Vec<&str> = from_vec.into_iter().collect();
+    assert_eq!(collected, vec!["a", "b"]);
+}
+
+#[test]
+fn lexer_and_parser_chain_through_plugin() {
+    let plugin = MockPlugin;
+    let doc = plugin.parse("  hello   world  ");
+
+    let BlockNode::Paragraph(p) = &doc.children[0] else {
+        panic!("expected a paragraph, got {:?}", doc.children[0]);
+    };
+    let InlineNode::Text(t) = &p.children[0] else {
+        panic!("expected a text node");
+    };
+    assert_eq!(t.content, "hello world");
+}
+
+#[test]
+fn registry_lookup_by_id_and_extension() {
+    let mut registry = SyntaxRegistry::new();
+    assert!(registry.is_empty());
+
+    registry.register(MockPlugin);
+    assert_eq!(registry.len(), 1);
+
+    let by_id = registry
+        .get("mock")
+        .expect("plugin should be findable by id");
+    assert_eq!(by_id.display_name(), "Mock Syntax");
+
+    let by_ext = registry
+        .detect_from_extension(".mk")
+        .expect("plugin should be findable by secondary extension");
+    assert_eq!(by_ext.id(), "mock");
+
+    assert!(registry.get("nonexistent").is_none());
+    assert!(registry.detect_from_extension(".unknown").is_none());
+
+    // Iteration order matches registration order.
+    let ids: Vec<&str> = registry.iter().map(|p| p.id()).collect();
+    assert_eq!(ids, vec!["mock"]);
+}
+
+#[test]
+fn registry_can_parse_through_trait_object() {
+    let mut registry = SyntaxRegistry::new();
+    registry.register(MockPlugin);
+
+    let plugin = registry.detect_from_extension(".mock").unwrap();
+    let doc = plugin.parse("a b c");
+    assert_eq!(doc.children.len(), 1);
+}
+
+/// Compile-time check: SyntaxRegistry must be Send + Sync so the LSP server
+/// can share it across handler tasks (SPEC.md §6.3).
+#[test]
+fn registry_is_send_and_sync() {
+    fn assert_send_sync<T: Send + Sync>() {}
+    assert_send_sync::<SyntaxRegistry>();
+}