Reduce CP HiCache L2 allocator scan cost

Host HiCache reservations were paying token-level free-slot scans when trying to preserve page contiguity. The allocator now keeps a lazy page-extent index so availability checks and contiguous-preferred allocations avoid materializing the full 220GB-equivalent free-slot metadata path.

The companion benchmark models steady-state L2 churn near full occupancy, including burn-in and historical node-size effects, so LPF/RDMA descriptor quality can be separated from ETE noise.

Constraint: CP HiCache host allocations are page-shaped, but existing callers may still read free_slots directly.
Rejected: Sort and scan free_slots on each alloc_contiguous_preferred call | measured ms-level CPU overhead on 220GB-equivalent metadata.
Rejected: Remove free_slots compatibility | storage/tests still rely on the public tensor surface.
Confidence: medium
Scope-risk: moderate
Directive: Do not reintroduce per-allocation full free_slots scans on HostKVCache; preserve page-extent metadata or benchmark before changing allocator shape.
Tested: Local py_compile for memory_pool_host.py, allocator benchmark, and related tests.
Tested: Local test_cp_hicache_allocator_bench.py 10 passed.
Tested: Remote g0034 test_hicache_controller_cp.py 67 passed; test_cp_hicache_allocator_bench.py 10 passed.
Tested: Remote 220GB-equivalent host_churn benchmark: contiguous path reduced from ms-level to ~30-292us p50 depending on fragmentation.
Not-tested: Full CUDA ETE run after allocator change.
Not-tested: Production long-run fragmentation behavior under live traffic.

test/registered/unit/benchmark/test_cp_hicache_allocator_bench.py

@@ -1,11 +1,14 @@
 import torch
 
 from benchmark.hicache.bench_cp_hicache_allocator_overhead import (
+    HostChurnBenchResult,
     StandaloneCPSharedPagedAllocator,
     StandaloneHostAllocator,
+    _bench_host_churn_case,
     _host_pages_from_gb,
     _make_host_free_slots,
     _make_page_compute_owners,
+    _page_run_lengths_from_token_slots,
     _parse_int_list,
 )
 
@@ -49,6 +52,65 @@ def test_host_random_fragmented_has_requested_size():
     assert torch.unique(free_slots).numel() == free_slots.numel()
 
 
+def test_page_run_lengths_from_token_slots_counts_lpf_descriptors():
+    page_size = 4
+    selected = torch.tensor(
+        [
+            *range(10 * page_size, 13 * page_size),
+            *range(20 * page_size, 22 * page_size),
+            *range(25 * page_size, 26 * page_size),
+        ],
+        dtype=torch.int64,
+    )
+
+    assert _page_run_lengths_from_token_slots(selected, page_size) == [3, 2, 1]
+
+
+def test_host_churn_case_reports_l2_run_quality():
+    result = _bench_host_churn_case(
+        impl="standalone",
+        method="contiguous",
+        total_pages=48,
+        request_pages=4,
+        page_size=8,
+        target_occupancy=0.75,
+        evict_pages=4,
+        eviction_pattern="random",
+        repeat=4,
+        warmup=1,
+        seed=7,
+    )
+
+    assert isinstance(result, HostChurnBenchResult)
+    assert result.bench == "host_churn"
+    assert result.repeat == 4
+    assert result.page_first_descriptors_per_op == 4
+    assert 0 < result.lpf_descriptor_ratio_mean <= 1
+    assert result.run_count_p50 >= 1
+    assert result.max_run_pages_mean >= 1
+
+
+def test_host_churn_prefill_node_pages_can_model_fragmented_free_chunks():
+    result = _bench_host_churn_case(
+        impl="standalone",
+        method="fifo",
+        total_pages=64,
+        request_pages=8,
+        page_size=4,
+        target_occupancy=0.75,
+        evict_pages=8,
+        eviction_pattern="random",
+        prefill_node_pages=1,
+        repeat=8,
+        warmup=2,
+        seed=11,
+    )
+
+    assert result.prefill_node_pages == 1
+    assert result.run_count_p50 > 1
+    assert result.lpf_descriptor_ratio_mean > 1 / result.request_pages
+
+
 def test_standalone_l1_allocator_reports_owner_lane_stats():
     allocator = StandaloneCPSharedPagedAllocator(
         physical_pages=4,
@@ -79,3 +141,25 @@ def test_standalone_l1_allocator_allocates_owner_matching_pages():
     logical_pages = (selected.view(-1, allocator.page_size)[:, 0] // allocator.page_size)
     selected_owners = torch.remainder(logical_pages - 1, allocator.cp_size).tolist()
     assert selected_owners == owners
+
+
+def test_host_churn_burnin_exposes_fragmented_evicted_nodes_after_cold_tail():
+    result = _bench_host_churn_case(
+        impl="standalone",
+        method="fifo",
+        total_pages=32,
+        request_pages=8,
+        page_size=4,
+        target_occupancy=0.75,
+        evict_pages=8,
+        eviction_pattern="random",
+        prefill_node_pages=1,
+        burnin=1,
+        repeat=1,
+        warmup=0,
+        seed=23,
+    )
+
+    assert result.burnin == 1
+    assert result.run_count_p50 > 1
+    assert result.lpf_descriptor_ratio_mean > 1 / result.request_pages