Fix RSS not dropping after delete_project: add mem_collect + immediate purge

- delete_project now calls cbm_mem_collect() after closing the database
  to return freed pages to OS immediately
- Set purge_delay=0 (was 1000ms) so mimalloc purges freed pages without delay
- Set purge_decommits=1 explicitly so macOS uses MADV_FREE_REUSABLE
  (immediate RSS accounting) instead of MADV_FREE (lazy, RSS stays high)

Before: deleting Linux kernel index left 8.1G RSS (2.1G dirty pages)
After: dirty pages purged immediately, RSS reflects actual usage

Author: DeusData

src/foundation/mem.c

@@ -116,8 +116,12 @@ void cbm_mem_init(double ram_fraction) {
         ram_fraction = DEFAULT_RAM_FRACTION;
     }
 
-    /* Reduce upfront memory: don't eagerly commit arenas */
+    /* Reduce upfront memory: don't eagerly commit arenas.
+     * Force decommit on purge (MADV_FREE_REUSABLE on macOS) so RSS
+     * drops immediately instead of staying high until memory pressure. */
     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 */
 
     cbm_system_info_t info = cbm_system_info();
     g_budget = (size_t)((double)info.total_ram * ram_fraction);

src/mcp/mcp.c

@@ -1554,6 +1554,7 @@ static char *handle_delete_project(cbm_mcp_server_t *srv, const char *args) {
     }
 
     cbm_pipeline_unlock();
+    cbm_mem_collect(); /* return freed pages to OS after closing database */
 
     yyjson_mut_doc *doc = yyjson_mut_doc_new(NULL);
     yyjson_mut_val *root = yyjson_mut_obj(doc);