fix epoch migration bug (#592)

Refactors the epoch system migration to prevent downgrade shenanigans
and jankery when pilots booted 3 -> 2 -> 3, or other similar
permutations. We use the version number in the `META` database of
`log/data.mdb` as our migration indicator now instead of the
(non-)existence of `log/data.mdb`.

pkg/noun/events.c

@@ -522,10 +522,10 @@ _ce_patch_verify(u3_ce_patch* pat_u)
   c3_zs ret_zs;
   c3_o  sou_o = c3n;  // south seen
 
-  if ( U3E_VERLAT != pat_u->con_u->ver_w ) {
+  if ( U3P_VERLAT != pat_u->con_u->ver_w ) {
     fprintf(stderr, "loom: patch version mismatch: have %"PRIc3_w", need %u\r\n",
                     pat_u->con_u->ver_w,
-                    U3E_VERLAT);
+                    U3P_VERLAT);
     return c3n;
   }
 
@@ -737,7 +737,7 @@ _ce_patch_compose(c3_w nor_w, c3_w sou_w)
 
     _ce_patch_create(pat_u);
     pat_u->con_u = c3_malloc(sizeof(u3e_control) + (pgs_w * sizeof(u3e_line)));
-    pat_u->con_u->ver_w = U3E_VERLAT;
+    pat_u->con_u->ver_w = U3P_VERLAT;
     pgc_w = 0;
 
     for ( i_w = 0; i_w < nor_w; i_w++ ) {

pkg/noun/version.h

@@ -11,18 +11,30 @@ typedef c3_w       u3v_version;
 #define U3V_VER3   3
 #define U3V_VERLAT U3V_VER3
 
-/* EVENTS
+/* PATCHES
  */
 
 typedef c3_w       u3e_version;
 
-#define U3E_VER1   1
-#define U3E_VERLAT U3E_VER1
+#define U3P_VER1   1
+#define U3P_VERLAT U3P_VER1
 
 /* DISK FORMAT
+ *
+ * synchronized between:
+ * - u3_disk->ver_w
+ * - version key in META table of data.mdb files
  */
 
-#define U3D_VER1   1
-#define U3D_VERLAT U3L_VER1
+#define U3D_VER1   1         // <= vere-v2
+#define U3D_VER2   2         // migrating to >= vere-v3
+#define U3D_VER3   3         // >= vere-v3
+#define U3D_VERLAT U3D_VER3
+
+/* EPOCH SYSTEM
+*/
+
+#define U3E_VER1   1
+#define U3E_VERLAT U3E_VER1
 
 #endif /* ifndef U3_VERSION_H */