Replace PAI vtable dispatch with direct calls

The pai_t interface implements C-style polymorphism via function pointers
to abstract over PAC and HPA. This abstraction provides no real benefit:
only two implementations exist, the dispatcher already knows which one to
use, and HPA stubs 2 of 5 operations. Remove the runtime dispatch in
favor of direct calls.

This commit:
- Promotes pac_alloc/expand/shrink/dalloc/time_until_deferred_work to
  external linkage and replaces the pai_t *self parameter with pac_t *pac.
- Promotes hpa_alloc/expand/shrink/dalloc/time_until_deferred_work to
  external linkage and replaces pai_t *self with hpa_shard_t *shard.
- Updates hpa_dalloc_batch's signature to take hpa_shard_t * directly
  and removes the hpa_from_pai container-of helper. Updates internal
  callers in hpa_alloc, hpa_dalloc, and hpa_sec_flush_impl.
- Drops the vtable assignments from pac_init() and hpa_shard_init().
- Replaces pai_alloc/dalloc/etc. dispatch in pa.c with direct calls.
  HPA expand and shrink (which are unconditional failure stubs) are
  skipped entirely for HPA-owned extents.
- Removes the pa_get_pai() helper.
- Updates tests in test/unit/hpa.c and test/unit/hpa_sec_integration.c
  to call hpa_alloc/dalloc/etc. directly.

The pai_t struct field stays as dead weight in pac_t and hpa_shard_t;
it is removed in the next commit along with pai.h itself.

No behavioral changes.

test/unit/hpa_thp_always.c

@@ -177,7 +177,7 @@ TEST_BEGIN(test_hpa_hugify_style_none_huge_no_syscall_thp_always) {
 	edata_t *edatas[NALLOCS];
 	ndefer_purge_calls = 0;
 	for (int i = 0; i < NALLOCS / 2; i++) {
-		edatas[i] = pai_alloc(tsdn, &shard->pai, PAGE, PAGE, false,
+		edatas[i] = hpa_alloc(tsdn, shard, PAGE, PAGE, false,
 		    false, false, &deferred_work_generated);
 		expect_ptr_not_null(edatas[i], "Unexpected null edata");
 	}