feat(diagnostics): add compiler-backed heml language service

Add hemx-lsp for stdio LSP diagnostics, completion, and hover while preserving HTML editor tooling for .heml files. Teach hemx-build to expose generated target and derive-known template context facts, including simple h-for locals, so editor help comes from build-owned facts instead of editor-only parsers. Wire VS Code/Cursor and Neovim documentation and extend the Workout exemplar with a real h-for plan loop for end-to-end proof.

req: diag/004

req: diag/005

req: diag/006

Cargo.lock

@@ -509,6 +509,8 @@ version = "0.1.0"
 dependencies = [
  "hemplate-core",
  "hemx-core",
+ "quote",
+ "syn",
 ]
 
 [[package]]
@@ -558,6 +560,14 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "hemx-lsp"
+version = "0.1.0"
+dependencies = [
+ "hemx-build",
+ "serde_json",
+]
+
 [[package]]
 name = "hemx-saas-example"
 version = "0.1.0"