M-INTEROCEPTION P3 (partial): descriptor-displacement drift-sensor primitive; self-anchor prerequisite flagged

Adds engram_geo_displacement(A, B, core_frac) — a read-only interoceptive
primitive that measures how far a neighborhood descriptor B has drifted from a
baseline A and decomposes it into GROWTH (periphery extends, core fixed) vs
CORRUPTION (the invariant core displaces). The core is the top core_frac of A's
members by centrality; per shared member (matched by id) the displacement is the
change in radial position (dist_centroid). Centroid separation (L2 + cosine) and
radius delta give the aggregate move. Pure function, no store mutation, no flag.

HONESTLY PARTIAL: a live self-drift reading needs a persisted SelfAnchor
baseline to compare "now" against, and no persisted self node / anchored
self-neighborhood exists in this store yet. The primitive takes an EXPLICIT
baseline so it is real and testable today; capturing a durable SelfAnchor and
wiring the ENGRAM_DRIFT_SENSOR live reading is a flagged follow-up. We do not
fabricate a self silently.

MEASURED on synthetic descriptors:
- GROWTH (periphery 0.50->0.90, core fixed): core_disp=0.000, periph_disp=0.400,
  centroid_sep=0.000, radius_delta=0.400.
- CORRUPTION (core 0.10->0.60, periphery fixed): core_disp=0.500,
  periph_disp=0.000, centroid_sep=0.566.
- Identity A vs A: zero drift.
The sensor discriminates cleanly (corruption core_disp >> growth core_disp).
ASan+UBSan clean.

lang/runtime/engram_geometry.h

@@ -158,6 +158,22 @@ GeoDescriptor* engram_geometry_descriptor(
 
 void engram_geo_free(GeoDescriptor* g);
 
+/* ── M-INTEROCEPTION P3: drift-sensor primitive (descriptor displacement) ────
+ * Read-only. GROWTH vs CORRUPTION split of how far B drifted from baseline A.
+ * See engram_geometry.c for the honesty note on the missing SelfAnchor. */
+typedef struct {
+    double centroid_sep;    /* L2 distance between centroids (same frame)        */
+    double centroid_cos;    /* 1 - cosine(centroidA, centroidB)                  */
+    double radius_delta;    /* |radiusA - radiusB| — neighborhood scale change   */
+    double core_disp;       /* mean radial displacement of the invariant core    */
+    double periph_disp;     /* mean radial displacement of the periphery         */
+    int    core_matched;    /* # core members matched by id across A,B           */
+    int    periph_matched;  /* # periphery members matched by id across A,B       */
+} GeoDisplacement;
+
+void engram_geo_displacement(const GeoDescriptor* a, const GeoDescriptor* b,
+                             double core_frac, GeoDisplacement* out);
+
 /* ═══════════════════════════════════════════════════════════════════════════
  * M10 — REIFICATION: densely co-wired relational neighborhoods crystallized into
  * FIRST-CLASS, PERSISTED store records (design doc §2; memory 885f5945). This is