* ZJIT: Share a single JITFrame across all C method frames

Replace the per-call JITFrame::new_cfunc allocation in gen_push_frame
with a sentinel value (ZJIT_JIT_RETURN_C_FRAME) stored in cfp->jit_return.
CFP_ZJIT_FRAME now returns a pointer to a single static rb_zjit_c_frame
(pc/iseq both NULL) when it sees the sentinel.

The CFP accessor is split into CFP_ZJIT_FRAME_P (predicate) and
CFP_ZJIT_FRAME (typed accessor that dereferences safely) so callers don't
have to know about the sentinel encoding. ISEQ frames are unchanged: they
still hand cfp->jit_return a heap-allocated JITFrame pointer.

* Move a pointer location for CFP_ZJIT_FRAME

A good poison value gives a recognizable fault address if someone
erroneously dereferences. Asserting the pointer is not poison shouldn't
be necessary, so move it to RUBY_DEBUG builds.

Return the pointer directly instead of a bool, for consistency with
CFP_ISEQ and CFP_PC. The pointer is truthy in if conditions so all
callers work unchanged.

Rename for consistency with ISEQ_BODY naming convention. Remove the
separate rb_cfp_has_iseq/rb_cfp_has_pc wrappers — CFP_ISEQ and CFP_PC
return NULL for missing values, which works directly in if conditions.

Per tenderlove's review, every cfp->iseq read should go through an
accessor that accounts for JITFrame. Rename the field to _iseq so
direct access causes a compile error, and rename the accessors to
drop the zjit prefix (rb_cfp_iseq, rb_cfp_has_iseq, rb_cfp_pc,
rb_cfp_has_pc).

Expose rb_zjit_jit_frame_update_references so vm.c can call into Rust
to relocate the iseq pointer during GC compaction, instead of manually
manipulating JITFrame fields in C. This keeps JITFrame field access
consolidated in Rust.

Make zjit.h the single source of truth for the JITFrame struct layout.
Remove the manual #[repr(C)] definition from jit_frame.rs and replace
it with a type alias to the bindgen-generated zjit_jit_frame.

Remove redundant rb_zjit_enabled_p checks from rb_zjit_cfp_pc and
rb_zjit_cfp_iseq since CFP_HAS_JIT_RETURN already checks it. Rewrite
rb_zjit_cfp_has_iseq and rb_zjit_cfp_has_pc as thin wrappers around
rb_zjit_cfp_iseq and rb_zjit_cfp_pc.