fix: memory not released after indexing (20GB+ RSS for 5MB data)

src/foundation/mem.c

@@ -13,7 +13,7 @@
 #include "foundation/constants.h"
 
 #define MAX_RAM_FRACTION 1.0
-#define DEFAULT_RAM_FRACTION 0.5
+#define DEFAULT_RAM_FRACTION 0.25
 #include <mimalloc.h>
 #include <stdatomic.h>
 #include <stdio.h>
@@ -122,6 +122,10 @@ void cbm_mem_init(double ram_fraction) {
     mi_option_set(mi_option_arena_eager_commit, 0);
     mi_option_set(mi_option_purge_decommits, SKIP_ONE);
     mi_option_set(mi_option_purge_delay, 0); /* immediate purge, no 1s delay */
+    /* Purge arenas aggressively (default multiplier is 10x purge_delay). */
+    mi_option_set(mi_option_arena_purge_mult, 1);
+    /* Reclaim pages from exited worker thread heaps on free. */
+    mi_option_set(mi_option_page_reclaim_on_free, 1);
 
     /* CBM_MEM_BUDGET_MB env override (memory analogue of CBM_WORKERS).
      * Lets users cap the budget directly without an enclosing cgroup —

src/foundation/system_info.c

@@ -298,7 +298,8 @@ int cbm_default_worker_count(bool initial) {
     cbm_system_info_t info = cbm_system_info();
     if (initial) {
         /* Use all cores for initial indexing — user is waiting */
-        return info.total_cores;
+        int cap = info.total_cores > 8 ? 8 : info.total_cores;
+        return cap;
     }
     /* Incremental: leave headroom for user's apps */
     int workers = info.perf_cores - SKIP_ONE;