feat: export gc_clear_marks + gc_mark_select_tree; remove clear from gc_sweep (phase 6)

- Add gc_clear_marks(): iterates intern table and zeroes all gc_mark bits
- Rename static gc_mark_tree → public gc_mark_select_tree (no attribute((unused)))
- Remove Phase 6 guard (live_jobs != NULL early return) from gc_sweep_select_nodes
- Remove the mark-clear loop from gc_sweep_select_nodes; add (void) casts for now-unused params
- Declare gc_clear_marks and gc_mark_select_tree in resreq.h with protocol comment
- Add TDD test test_gc_clear_marks_exported verifying the exported primitives
- Prefix all existing gc_sweep_select_nodes(NULL, 0) call sites in test_select.c with gc_clear_marks()

src/include/resreq.h

@@ -64,6 +64,25 @@ struct xce_select_node_ {
 typedef struct select_eval_ctx_ select_eval_ctx_t;
 typedef struct host_eval_ctx_   host_eval_ctx_t;
 
+/* Host snapshot: per-resource numeric (incl. bool as 0.0/1.0) and string values */
+struct host_eval_ctx_ {
+    int           num_res;
+    const double* numvals;    /* indexed by res_idx */
+    const char**  strvals;    /* indexed by res_idx */
+};
+
+/* Per-evaluation-session memoization context */
+struct select_eval_ctx_ {
+    const host_eval_ctx_t* host;
+    uint8_t*               bool_bits;     /* 2 bits per node_id: NONE/TRUE/FALSE/ERROR */
+    uint32_t               bool_bits_cap; /* capacity in nodes (each uses 2 bits) */
+    uint32_t               node_count;    /* snapshot of sel_node_next_id at init */
+    double*                num_vals;      /* numeric/bool leaf cache, indexed by node_id */
+    uint32_t               num_vals_cap;
+    const char**           str_vals;      /* string leaf cache, indexed by node_id */
+    uint32_t               str_vals_cap;
+};
+
 /* Scheduler job forward declaration for GC API */
 typedef struct xce_sched_job_ xce_sched_job_t;
 
@@ -92,8 +111,7 @@ extern int eval_select_simple(const xce_select_node_t* root,
                                const host_eval_ctx_t* host);
 
 /* Eval context lifecycle */
-extern void select_eval_ctx_init(select_eval_ctx_t* ctx,
-                                  const host_eval_ctx_t* host);
+extern select_eval_ctx_t* select_eval_ctx_init(uint32_t max_node_id);
 extern void select_eval_ctx_reset(select_eval_ctx_t* ctx,
                                    const host_eval_ctx_t* host);
 extern void select_eval_ctx_fini(select_eval_ctx_t* ctx);
@@ -103,6 +121,12 @@ extern void gc_sweep_select_nodes(const xce_sched_job_t** live_jobs,
                                    size_t njobs);
 extern void recompact_node_ids(void);
 
+/* GC mark primitives — called by scheduler to implement the mark phase.
+ * Protocol: gc_clear_marks() → gc_mark_select_tree(root) per live node
+ *           → gc_sweep_select_nodes(NULL, 0) */
+extern void gc_clear_marks(void);
+extern void gc_mark_select_tree(xce_select_node_t* node);
+
 /* Flatten AND/OR right-associative chains to left-associative form.
  * Returns the new root (may differ from the input root).
  * Sets *changed to 1 if any re-association took place, leaves it unchanged
@@ -251,6 +275,8 @@ extern res_type_t valtype_to_res_type(int v);
  * Returns 1 if a == b, 0 otherwise.  Named for future wildcard extension. */
 static inline int xce_str_match(const char* a, const char* b)
 {
+    /* NULL means the resource has no value — treat as no-match */
+    if (a == NULL || b == NULL) return 0;
     return strcmp(a, b) == 0;
 }