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.
claude@clouddev1
f0b2043a39