walker: add Subgrammar node + recursion-depth cap (ADR-0026 step 1)

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.

src/friendly/keys.rs

@@ -183,6 +183,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.custom.bind_type_mismatch", &["found", "expected"]),
     ("parse.custom.change_column_flags_exclusive", &[]),
     ("parse.custom.create_table_needs_pk", &[]),
+    ("parse.custom.expression_too_deep", &[]),
     ("parse.custom.insert_form_a_missing_values", &["columns"]),
     ("parse.custom.on_action_specified_twice", &["target"]),
     ("parse.custom.replay_path_expected", &[]),