feat: implement sched_meet_host_selection using eval_select_simple (6-5)

Replace the always-return-1 stub with a real implementation that:
- Returns 1 immediately when s == NULL or s->root == NULL (no constraint)
- Builds a host_eval_ctx_t from w->resvals and calls eval_select_simple
- Uses a zero-resource stack context when resvals.caps == 0 so constant
  sub-expressions (SEL_TRUE, SEL_FALSE, SEL_ANY) still evaluate correctly
- On OOM (build_host_eval_ctx returns NULL), conservatively passes the host
- Maps eval_select_simple result: >0 → 1 (match), <=0 → 0 (no-match)

Add #define UNIT_TEST 1 to test.c, remove redundant #ifdef guard, and
add three new TDD tests covering NULL select, SEL_TRUE, and SEL_FALSE.
All 6 tests pass.

src/scheduler/src/resource.c

@@ -179,8 +179,35 @@ free_host_eval_ctx(host_eval_ctx_t* ctx)
 int sched_meet_host_selection(xce_scheduler_t* sched, xce_sched_worker_t* w,
                               xce_select_t* s)
 {
-    /* FIXME: add  real work later */
-    return 1;
+    (void)sched;
+
+    if (s == NULL || s->root == NULL) return 1;
+
+    int num_res = (int)w->resvals.caps;
+
+    /* When num_res == 0, build_host_eval_ctx returns NULL (no resource slots).
+     * Use an empty stack-allocated context so eval_select_simple can still
+     * evaluate constant sub-expressions (SEL_TRUE, SEL_FALSE, SEL_ANY). */
+    host_eval_ctx_t  empty_ctx = { .num_res = 0, .numvals = NULL, .strvals = NULL };
+    host_eval_ctx_t* hctx;
+
+    if (num_res <= 0) {
+        hctx = &empty_ctx;
+    } else {
+        hctx = build_host_eval_ctx(w, num_res);
+        if (hctx == NULL) {
+            /* OOM: conservatively pass host */
+            return 1;
+        }
+    }
+
+    int rc = eval_select_simple(s->root, hctx);
+    if (hctx != &empty_ctx) {
+        free_host_eval_ctx(hctx);
+    }
+
+    /* eval_select_simple: 1=match, 0=no-match, <0=error (treat as no-match) */
+    return (rc > 0) ? 1 : 0;
 }
 
 static int sched_check_shared_satisfy_(xce_scheduler_t* sched, xvec_t* total,

src/scheduler/testsuite/test.c

@@ -1,3 +1,4 @@
+#define UNIT_TEST 1
 #include <argp.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -122,9 +123,77 @@ test_sel_optimize_set_at_scheduler_startup(void)
     sel_optimize_set(1);
     TEST_ASSERT_EQUAL_INT(1, sel_get_optimize_level());
     /* reset for clean test isolation */
-#ifdef UNIT_TEST
     sel_optimize_test_reset();
-#endif
+}
+
+static void
+test_meet_host_selection_null_select_passes(void)
+{
+    /* A NULL select pointer means no expression — always pass */
+    xce_scheduler_t sched;
+    memset(&sched, 0, sizeof(sched));
+    xce_sched_worker_t w;
+    memset(&w, 0, sizeof(w));
+    zarray_init(&w.resvals, 4, NULL);
+
+    int rc = sched_meet_host_selection(&sched, &w, NULL);
+    TEST_ASSERT_EQUAL_INT(1, rc);
+
+    zarray_fini(&w.resvals);
+}
+
+static void
+test_meet_host_selection_always_true_passes(void)
+{
+    xce_scheduler_t sched;
+    memset(&sched, 0, sizeof(sched));
+    sel_optimize_set(1);
+    sel_node_table_init();
+
+    xce_sched_worker_t w;
+    memset(&w, 0, sizeof(w));
+    zarray_init(&w.resvals, 4, NULL);
+
+    /* SEL_TRUE root — must pass any host */
+    xce_select_node_t* root = makeSelectNode(SEL_TRUE, 0, 0);
+    TEST_ASSERT_NOT_NULL(root);
+
+    xce_select_t sel = { .root = root };
+    int rc = sched_meet_host_selection(&sched, &w, &sel);
+    TEST_ASSERT_EQUAL_INT(1, rc);
+
+    zarray_fini(&w.resvals);
+    gc_clear_marks();
+    gc_sweep_select_nodes(NULL, 0);
+    sel_node_table_fini();
+    sel_optimize_test_reset();
+}
+
+static void
+test_meet_host_selection_always_false_fails(void)
+{
+    xce_scheduler_t sched;
+    memset(&sched, 0, sizeof(sched));
+    sel_optimize_set(1);
+    sel_node_table_init();
+
+    xce_sched_worker_t w;
+    memset(&w, 0, sizeof(w));
+    zarray_init(&w.resvals, 4, NULL);
+
+    /* SEL_FALSE root — must fail every host */
+    xce_select_node_t* root = makeSelectNode(SEL_FALSE, 0, 0);
+    TEST_ASSERT_NOT_NULL(root);
+
+    xce_select_t sel = { .root = root };
+    int rc = sched_meet_host_selection(&sched, &w, &sel);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    zarray_fini(&w.resvals);
+    gc_clear_marks();
+    gc_sweep_select_nodes(NULL, 0);
+    sel_node_table_fini();
+    sel_optimize_test_reset();
 }
 
 #define TEST(x)                                                                \
@@ -143,6 +212,9 @@ int main(int argc, char* argv[])
     TEST(test_build_host_eval_ctx_float);
     TEST(test_scheduler_t_has_atomic_gc_fields);
     TEST(test_sel_optimize_set_at_scheduler_startup);
+    TEST(test_meet_host_selection_null_select_passes);
+    TEST(test_meet_host_selection_always_true_passes);
+    TEST(test_meet_host_selection_always_false_fails);
 
     return UNITY_END();
 }