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/strings/en-US.yaml

@@ -372,6 +372,11 @@ parse:
     replay_path_expected: "expected a path after `replay`"
     create_table_needs_pk: |-
       tables need at least one column. Add `with pk` for a default `id INTEGER PRIMARY KEY`, or `with pk <name>(<type>)` to choose. Use a comma-separated list for compound primary keys.
+    # ADR-0026 §1: the recursion-depth guard on the
+    # WHERE-expression grammar. Input nested past the cap
+    # (`((((…))))`) stops here with a friendly error instead
+    # of overflowing the parser stack.
+    expression_too_deep: "expression nested too deeply"
     on_action_specified_twice: "`on {target}` specified twice"
     change_column_flags_exclusive: "`--force-conversion` and `--dont-convert` are mutually exclusive — pick one."
     unknown_type: "unknown type '{found}' (expected one of: {expected})"