Consolidate arena_* header split into arena.h

arena_types.h + arena_structs.h + arena_externs.h merged into arena.h,
keeping the three logical sections (TYPES / STRUCTS / EXTERNS) with
explicit dividers.  arena_inlines_a.h and arena_inlines_b.h stay
separate; arena_inlines_b.h now carries a comment explaining why
merging the two would reintroduce a real #include cycle through
tcache_inlines.h -> arena_choose (the asymmetric cycle-breaker).

arena_decay_constants.h (new): minimal header for
ARENA_DECAY_NTICKS_PER_UPDATE.

test/integration/extent.c

@@ -2,7 +2,7 @@
 
 #include "test/extent_hooks.h"
 
-#include "jemalloc/internal/arena_types.h"
+#include "jemalloc/internal/arena.h"
 
 static void
 test_extent_body(unsigned arena_ind) {

test/unit/san_bump.c

@@ -1,7 +1,7 @@
 #include "test/jemalloc_test.h"
 #include "test/arena_util.h"
 
-#include "jemalloc/internal/arena_structs.h"
+#include "jemalloc/internal/arena.h"
 #include "jemalloc/internal/san_bump.h"
 
 static extent_hooks_t *san_bump_default_hooks;

test/unit/stats.c

@@ -1,6 +1,6 @@
 #include "test/jemalloc_test.h"
 
-#include "jemalloc/internal/arena_structs.h"
+#include "jemalloc/internal/arena.h"
 
 #define STRINGIFY_HELPER(x) #x
 #define STRINGIFY(x) STRINGIFY_HELPER(x)