fix(mem): reconcile peak RSS with live current RSS (Linux statm vs ru_maxrss)

src/foundation/mem.c

@@ -212,6 +212,17 @@ size_t cbm_mem_rss(void) {
 size_t cbm_mem_peak_rss(void) {
     size_t peak_rss = 0;
     mi_process_info(NULL, NULL, NULL, NULL, &peak_rss, NULL, NULL, NULL);
+    /* Peak RSS is by definition >= current RSS. On Linux cbm_mem_rss() reads the
+     * live /proc/self/statm value (page-granular), while mimalloc's peak_rss
+     * comes from getrusage's ru_maxrss (KB-granular, and it can lag the live
+     * statm read by a few pages). Reading the two sources independently lets a
+     * fresh current read momentarily exceed the reported peak, breaking the
+     * peak >= current invariant. Reconcile them: the true peak is at least the
+     * current RSS. (Not observable on macOS, where both come from mimalloc.) */
+    size_t current = cbm_mem_rss();
+    if (current > peak_rss) {
+        peak_rss = current;
+    }
     if (peak_rss > 0) {
         return peak_rss;
     }

tests/test_mem.c

@@ -180,10 +180,23 @@ TEST(mem_rss_positive) {
 
 TEST(mem_peak_rss_gte_rss) {
     cbm_mem_init(0.5);
+    /* peak >= current RSS is definitional. Regression guard for the Linux
+     * statm-vs-ru_maxrss source mismatch: cbm_mem_rss() reads the live
+     * /proc/self/statm value (page-granular) while mimalloc's peak comes from
+     * getrusage ru_maxrss (KB-granular, and it lags), so a live current read
+     * could momentarily exceed the reported peak by a few pages and break the
+     * invariant. cbm_mem_peak_rss() now reconciles the two sources. Touch a
+     * fresh buffer so the check runs against a non-trivial live current read.
+     * (Linux-only bug — macOS reads both from mimalloc; it flaked on the
+     * Linux/ARM CI leg, which is the authoritative reproduction tier.) */
+    size_t n = 32 * 1024 * 1024;
+    char *p = (char *)malloc(n);
+    ASSERT_NOT_NULL(p);
+    memset(p, 0xBE, n); /* fault in all pages so current RSS is non-trivial */
     size_t rss = cbm_mem_rss();
     size_t peak = cbm_mem_peak_rss();
-    /* Peak must be >= current RSS */
     ASSERT_GTE(peak, rss);
+    free(p);
     PASS();
 }