eval, the number-one conformance lever, and ShadowRealm

Indirect and global-scope eval land, followed by ShadowRealm, both as runtime structures rather than native recursion.

76169428: "implement eval (indirect / global-scope) — the #1 conformance lever". 97e07a67 adds ShadowRealm. Both are cheap in an explicit-frame VM: a new realm is a new set of intrinsics and a new global, not a new native stack.