feat: implement Phase 4d const-fold transform for select AST

Adds sel_const_fold_pass() — a single-pass bottom-up const-fold that
runs after the Phase 4c flatten loop in parse_select():

  4d-1  EQ/NE with either operand SEL_ANY → SEL_TRUE / SEL_FALSE
        (checks both child slots — commutativity sort makes SEL_ANY
        position non-deterministic after makeSelectNode interning)
  4d-2  Identity laws: TRUE&&X→X, X&&TRUE→X, FALSE||X→X, X||FALSE→X
  4d-3  Annihilation laws: FALSE&&X→FALSE, TRUE||X→TRUE (symmetric)
  4d-4  Complement laws: !TRUE→FALSE, !FALSE→TRUE

Memory semantics:
  level==0 (API server): frees orphaned nodes via free_select_node /
    sel_free_tree; OOM from makeSelectNode returns original node safely
  level>0  (CSE/scheduler): never manually frees; annihilation folds
    return the already-interned left/right singleton directly (no
    redundant makeSelectNode call); OOM guard unconditional so CSE
    mode also degrades gracefully instead of returning NULL

9 new TDD tests added to test_select.c (RED→GREEN verified).
All 166 tests pass (157 prior + 9 Phase 4d).

src/lib/testsuite/test_resmgr_fixture.c

@@ -0,0 +1,58 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include <rtti.h>
+
+#include "kv.h"
+#include "resreq.h"
+#include "xce/types.h"
+#include "test_resmgr_fixture.h"
+
+/* Internal helper to add one resource to the test resmgr */
+static void add_resource(var resmgr, const char* name, int idx, int valtype)
+{
+    xce_resource_t* r = calloc(1, sizeof(*r));
+    if (r == NULL) return;
+    r->name    = strdup(name);
+    r->idx     = idx;
+    r->valtype = valtype;
+    key_add(resmgr, (void*)r->name, r);
+}
+
+var make_test_resmgr(void)
+{
+    kv_params_t params = {
+        .nslots     = 64,
+        .keyops     = &string_key_ops,
+        .datakiller = NULL,   /* we free resources manually in free_test_resmgr */
+    };
+    var resmgr = makeInstance(&kvtab_type, &params);
+    if (resmgr == NULL) return NULL;
+
+    add_resource(resmgr, "type",  TEST_RES_TYPE_IDX,  XCE_VAL_STRING);
+    add_resource(resmgr, "mem",   TEST_RES_MEM_IDX,   XCE_VAL_NUMERIC);
+    add_resource(resmgr, "gpu",   TEST_RES_GPU_IDX,   XCE_VAL_BOOLEAN);
+    add_resource(resmgr, "ncpus", TEST_RES_NCPUS_IDX, XCE_VAL_NUMERIC);
+    add_resource(resmgr, "arch",  TEST_RES_ARCH_IDX,  XCE_VAL_STRING);
+
+    return resmgr;
+}
+
+void free_test_resmgr(var resmgr)
+{
+    if (resmgr == NULL) return;
+
+    /* Free each xce_resource_t we allocated (datakiller=NULL means kvtab
+     * won't free them, so we must do it here). */
+    const char* names[] = { "type", "mem", "gpu", "ncpus", "arch", NULL };
+    for (int i = 0; names[i] != NULL; i++) {
+        xce_resource_t* r = NULL;
+        int rc = key_find(resmgr, (void*)names[i], (void**)&r);
+        if (rc > 0 && r != NULL) {
+            free(r->name);
+            free(r);
+        }
+    }
+
+    destroy(resmgr);
+}