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_background_thread.c

@@ -20,7 +20,7 @@ TEST_BEGIN(test_hpa_background_thread_a0_initialized) {
 
 	arena_t *a0 = arena_get(TSDN_NULL, 0, false);
 	expect_ptr_ne(a0, NULL, "");
-	bool deferral_allowed = a0->pa_shard.hpa_shard.opts.deferral_allowed;
+	bool deferral_allowed = a0->pa_shard.hpa.opts.deferral_allowed;
 	expect_true(deferral_allowed,
 	    "Should have deferral_allowed option enabled for arena #0");
 }