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.