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

hemx-lsp/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "hemx-lsp"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+hemx-build = { path = "../hemx-build" }
+serde_json = "1"