docs(editor): define hemplate highlight boundary

Document the repo-owned .heml highlighting overlay tokens and exclude diagnostics/completion/formatting/build validation from highlighting authority.

req: diagnostics/004

req: diagnostics/008

docs/editor-support.md

@@ -67,6 +67,28 @@ The one-shot command prints a JSON object shaped like LSP
 The diagnostic payload comes from `hemx-build`; editor integrations should display
 it as-is instead of recreating the rule.
 
+## Highlighting boundary
+
+Repo-owned `.heml` highlighting is an HTML overlay, not a new language. Normal
+HTML highlighting owns tags, attributes, strings, comments, folding, and tag
+matching. The hemplate overlay may highlight only documented syntax tokens from
+`docs/hemplate-syntax.md`:
+
+- escaped text delimiters and expression regions: `{+` and `+}`;
+- trusted/rendered HTML delimiters and expression regions: `{+=` and `=+}`;
+- dynamic attribute prefixes such as `+class`, `+disabled`, and `+aria-label`;
+- structural directives: `h-if`, `h-for`, `h-key`, `h-match`, and `h-case`;
+- hemx facts recorded as ordinary attributes: `data-hemx-root`,
+  `data-hemx-slot`, `data-hemx-form`, `data-hemx-handle`, and other checked
+  `data-hemx-*` authoring attributes.
+
+Highlighting must not own diagnostics, completion, hover, formatting, Rust
+expression parsing, selector behavior, generated Rust facts, or build
+validation. Those remain with `hemx-build`, `hemx-lsp`, normal HTML tooling, and
+Rust tooling. Repo tests for highlighting should therefore be fixture/query tests
+for captures over these token classes; provider packaging or visual editor smoke
+is a separate release slice and cannot become syntax authority. req: diagnostics/004 req: diagnostics/008
+
 ## VS Code and Cursor
 
 Use the shared repo extension in `editors/vscode-hemx` for VS Code and Cursor.