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>
gffranco