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

README.md

@@ -13,6 +13,10 @@ The v1 story now has tutorial, recipe, diagnostics, and stability docs; see
 Template authoring: `.heml` is HTML plus a small hemplate overlay for escaped
 text, trusted HTML, dynamic attributes, Rust-shaped control directives, generated
 slots/forms/handles, and keyed partial targets. See `docs/hemplate-syntax.md`.
+Editor setup for VS Code, Cursor, and Neovim lives in `docs/editor-support.md`;
+VS Code/Cursor share the repo extension in `editors/vscode-hemx`, while all
+editors keep normal HTML/tree-sitter highlighting and layer `hemx-build`
+diagnostics on top.
 
 Local checkout note: until the hemplate crates are published, this repository
 expects `hemplate` checked out next to `hemx` as `../hemplate/hemplate`. The app