Prove draft HiCache mirrors target valid-tail pages

Draft HiCache already piggybacks target CP reservations, but the controller coverage only exercised page-aligned nodes. The new tests pin the intended contract for valid-tail nodes: target and draft reserve/load the same padded physical page while scheduler-visible load results keep only valid logical locs.

Constraint: Draft KV is a target mirror; it must not choose an independent valid-only host path.
Rejected: Re-enable draft partial-current reuse in this slice | current safe contract keeps EAGLE/NextN cache-hit draft on full materialization until same-layer padded visibility is proven.
Confidence: medium
Scope-risk: narrow
Directive: Keep draft and target host metadata coupled; do not add draft-only capacity or prefetch decisions.
Tested: local py_compile for test_hicache_controller_cp.py.
Tested: remote g0034 test_hicache_controller_cp.py: 59 passed, 3 warnings.
Not-tested: CUDA E2E runtime for this commit.
Co-authored-by: OmX <omx@oh-my-codex.dev>

docs/advanced_features/nsa_prefill_cp_page_aligned_cache_contract.md

@@ -697,6 +697,9 @@ Current state:
   `draft_host_indices` are present.
 - Draft partial-current reuse/prefetch has been disabled or pushed to older
   materialization paths in some cases to avoid EAGLE hangs.
+- Existing draft HiCache controller tests cover page-aligned write/load/evict,
+  but they do not yet prove that a valid-tail node mirrors the target padded
+  physical span while exposing only the valid length.
 
 Correction:
 
@@ -712,6 +715,15 @@ Tests:
   `owned_positions` and padded physical length.
 - EAGLE one-layer path does not prefetch or consume an unmasked padded tail.
 
+Implemented C9 slice:
+
+- Added target/draft write coverage for a valid-tail suffix.  Target and draft
+  reserve the same padded local positions and transfer the same physical device
+  span while metadata keeps the valid `logical_len`.
+- Added target/draft load coverage for a valid-tail host hit.  Both pools load
+  the padded physical page, but `load_cp()` returns only valid logical locs to
+  the scheduler.
+
 ### C10. Disaggregated transfer has not been audited for valid/padded split
 
 Current state: