EXPERIMENT: invocation control resolves at runtime

ISHIKAWA: why did wraps_body need compile-time knowledge? Because the wrapper
called the target directly. If the wrapper calls through the seam instead, the
seam can call the body itself, and a construct bound after the build decides
how and whether to invoke it.

PREDICTIONS AND RESULTS
  P1 wrap becomes runtime-bindable                  TRUE   body x3 -> 21,
                                                           never invoked -> 111
  P2 codegen shrinks                                TRUE   5042 -> 4977
  P3 cost 5-10% from an indirect call on every fn   TRUE   0.36s -> 0.39s, ~8%
  P4 zero-param fns break on the empty struct       TRUE   empty struct is a GNU
                                                           extension, empty init
                                                           is C23. Fixed with a
                                                           char field.
  P5 fixpoint holds                                 TRUE

PROCESS FAILURE worth recording: my first patch silently did not apply because
I dropped the assert on the string replacement. The build then failed with
"undeclared identifier __thunk_noargs", which I nearly attributed to the
empty-struct prediction. The guard that would have caught it existed and I
removed it -- the same shape as every other defect found tonight.

Removed: declare_wrap, decorator_wrap, cg_wrap_target, cg_wrap_construct,
params_to_call_args, and the wraps_body scanner branch.

prohibits_outside is now the ONLY construct kind left at compile time, and it
cannot move: a #error has no runtime.

lang/runtime/el_runtime.h

@@ -888,7 +888,8 @@ el_val_t  engram_age_field_catchup(void);
 el_val_t  engram_chrono_persist_tick(void);
 el_val_t  engram_chrono_tick(void);
 el_val_t  engram_boundary_beat(el_val_t op_name, el_val_t construct);
-el_val_t  el_seam_run(el_val_t fn_name, el_val_t phase, el_val_t result);  /* runtime construct seam */  /* API-reshape decorator-seam auto-emit; construct = the decorator that caused the beat */
+el_val_t  el_seam_run(el_val_t fn_name, el_val_t phase, el_val_t result);  /* runtime construct seam */
+el_val_t  el_seam_wrap(el_val_t fn_name, el_val_t (*body)(void*), void* env);  /* runtime invocation control */  /* API-reshape decorator-seam auto-emit; construct = the decorator that caused the beat */
 el_val_t  engram_self_anchor_capture(void);
 el_val_t  engram_self_drift_json(void);
 el_val_t  engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction);