claude@clouddev1
f0b2043a39
New `Node::Subgrammar(&'static Node)` variant lets a named static grammar fragment recurse through a reference — `Seq` / `Choice` embed children by value and cannot close a cycle, but a `&'static Node` can point back at an enclosing fragment. This is the mechanism the stratified WHERE-expression grammar (ADR-0026 §2) recurses through. The walker counts active Subgrammar frames in `WalkContext::subgrammar_depth` and refuses past `MAX_SUBGRAMMAR_DEPTH` (64), surfacing a friendly `parse.custom.expression_too_deep` error instead of a stack overflow. Depth is saved/restored per frame so a speculatively-walked-then-rolled-back Choice branch leaves no residue. No grammar references the node yet; covered by walker unit tests with a small recursive `( x )` test grammar.