M-INTEROCEPTION P2: chronoception — age the activation field by measured wall-clock delta (ENGRAM_CHRONOCEPTION, default OFF)

The felt passage of time is the cooling of the activation field, not a tick
count and not an elapsed-seconds readout. engram_age_field(delta_ms) cools the
field (working_memory_weight + background_activation) by the caller's MEASURED
wall-clock delta with a pure exponential exp(-dt/TC) — no per-call floor — so it
is exactly scale-invariant: N ticks summing to the same elapsed time produce the
same total cooling. It returns the cooling MAGNITUDE (1-exp(-dt/TC), a bounded
[0,1) drift signal), never elapsed seconds.

Reboot = anesthesia: a global last-tick wall-clock stamp is persisted to a
sidecar (chrono_last_tick) in the data dir. engram_age_field_catchup() reads it
on boot, applies ONE cooling for the whole unconscious gap, refreshes the stamp,
and reports the magnitude — timestamps are bookkeeping to COMPUTE the drift,
never the felt signal. TC env-tunable via ENGRAM_CHRONO_TC (default 3600s).

All inert unless ENGRAM_CHRONOCEPTION is set → OFF path byte-identical.

MEASURED on a copy (throwaway HOME, TC=3600s):
- Cooling scales with dt, matching 1-exp(-dt/TC) to 1e-6: dt=600s->0.1535,
  1800s->0.3935, 3600s->0.6321, 7200s->0.8647.
- Scale-invariance EXACT: age(dt) once vs age(dt/N) N times gives identical
  field sum (|delta|=0.0) for N=2, 10, 100.
- Reboot catch-up over a 1h gap cooled the field 0.60->0.4415 in one shot,
  magnitude 0.6321, bounded in [0,1).
- Flag OFF: age & catchup return 0, field untouched (sum 1.2).
ASan+UBSan clean.

lang/runtime/el_runtime.h

@@ -623,6 +623,9 @@ el_val_t  engram_scan_nodes_json(el_val_t limit, el_val_t offset);
 el_val_t  engram_scan_nodes_by_type_json(el_val_t node_type, el_val_t limit, el_val_t offset);
 el_val_t  engram_scan_nodes_emb_json(el_val_t limit, el_val_t offset);
 el_val_t  engram_consolidate_permanence(el_val_t node_id);
+el_val_t  engram_age_field(el_val_t delta_ms);
+el_val_t  engram_age_field_catchup(void);
+el_val_t  engram_chrono_persist_tick(void);
 el_val_t  engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction);
 el_val_t  engram_activate_json(el_val_t query, el_val_t depth);
 el_val_t  engram_stats_json(void);