Address review feedback on server selection (PR #5508)

- Fix truncation: use uint64_t modulo instead of unsigned int cast
  when computing target from total_weight (prevents skewed selection
  when total_weight > UINT_MAX)
- Remove unused #include <cstdlib>
- Add docstring note about max_latency_us=0 semantics: in production
  this means "use thread default", this extraction treats it as "no
  limit" — callers should resolve defaults before populating the struct
- Remove trailing "fix" comment from test

include/ServerSelection.h

@@ -55,6 +55,11 @@ struct ServerCandidate {
  *   - current_latency_us <= max_latency_us (or max_latency_us == 0)
  *   - current_repl_lag <= max_repl_lag (or max_repl_lag == 0)
  *
+ * @note In production, max_latency_us == 0 on a per-server basis means
+ *       "use the thread default max latency." This extraction treats 0
+ *       as "no limit" for simplicity. Callers should resolve defaults
+ *       before populating the ServerCandidate.
+ *
  * @return true if the candidate is eligible.
  */
 bool is_candidate_eligible(const ServerCandidate &candidate);

lib/ServerSelection.cpp

@@ -7,7 +7,6 @@
  */
 
 #include "ServerSelection.h"
-#include <cstdlib>
 
 bool is_candidate_eligible(const ServerCandidate &c) {
 	if (c.status != SERVER_ONLINE) {
@@ -51,7 +50,8 @@ int select_server_from_candidates(
 	// LCG: next = (a * seed + c) mod m  (Numerical Recipes parameters)
 	unsigned int rng_state = random_seed;
 	rng_state = rng_state * 1664525u + 1013904223u;
-	int64_t target = (int64_t)(rng_state % (unsigned int)total_weight) + 1;
+	// Use 64-bit modulo to avoid truncation when total_weight > UINT_MAX
+	int64_t target = (int64_t)(rng_state % (uint64_t)total_weight) + 1;
 
 	// Second pass: weighted selection
 	int64_t cumulative = 0;

test/tap/tests/unit/server_selection_unit-t.cpp

@@ -216,7 +216,7 @@ int main() {
 	test_weighted_distribution();        // 1
 	test_determinism();                  // 1
 	test_mixed_eligibility();            // 1
-	// Total: 1+12+1+1+1+1+1+1+1+1 = 21... fix
+	// Total: 1+12+1+1+1+1+1+1+1+1 = 21
 
 	test_cleanup_minimal();
 	return exit_status();