Stack Runtime
src/parser/stack.zig keeps states and values as parallel unmanaged ArrayLists. push appends to both; pop removes from both (restoring the state on value-stack underflow, which cannot happen when callers respect the depth checks); popMany(n) asserts n < depth so the bottom state always survives for recovery anchoring. clearRetainingCapacity between parses is what makes parser reuse allocation-free in steady state.