feat(vm-jit): native-call ABI slice 4 core — mutual-recursion group compile

The "declare a cycle before defining" mechanism for native mutual recursion.

- New `JitInstr::CallGroup { group_index, dst, args }`: a mutually-recursive call
  to a co-compiled group member (by index), since a sibling's id isn't minted
  until the whole cycle is declared.
- `NativeModule::compile_recursive_group(&[JitFunction])`: declares every member's
  FuncId first, then builds+defines each with CallGroup resolving to the declared
  ids, then finalizes once. Returns one CompiledId per member.
- The entry depth guard now fires for group members too (has_call_self ||
  has_call_group) — mutual native recursion consumes the host C stack just like
  self-recursion, so the cap keeps a cycle from overflowing it (the failure mode
  is a crash). CallGroup is non-chaining (re-run-from-top deopt) like CallSelf,
  with its own callee-sized scratch slots (discarded on bail).
- Extracted `push_compiled_abi_signature` so the CompiledAbi is defined once.

Unit test `native_mutual_recursion_group_computes_and_caps_depth`: is_even/is_odd
co-compiled, run natively (correct), and deep mutual recursion bails cleanly at
the cap (no host-stack overflow). vm-jit 83/0, differential 33/0, default build OK.

Next: reg-vm wiring (SCC detection + dispatch + translate CallGroup emission) to
run mutual recursion natively from rss source end-to-end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang