Keep CP HiCache draft KV invisible until joint readiness

CP HiCache now treats draft KV as a strict target-owned payload through pending write visibility, host eviction, and state-buffer registration. Host metadata created before async D2H ack is no longer request-visible, so match_prefix cannot select an in-flight host node. Draft host eviction now fails before target cleanup when draft metadata is missing, and prefill/decode share one helper for draft NSA state buffers so shared-KV mode cannot silently skip mismatched draft state.

Constraint: CP shared KV + HiCache + EAGLE/MTP must not expose target-only host hits or skipped draft state as valid cache hits

Rejected: Rely on event-loop ordering and lock_ref to hide in-flight writes | match_prefix does not consult lock_ref and can observe host_len/cp_hicache directly

Rejected: Keep draft state mismatch as debug-only skip | it can poison speculative acceptance while looking like a successful cache hit

Confidence: high

Scope-risk: moderate

Directive: Do not reintroduce silent draft/target fallback in CP shared-KV HiCache paths; malformed strong-sync metadata should fail fast

Tested: python -m py_compile targeted modified files

Tested: remote g0034 container pytest test/registered/unit/mem_cache/test_cp_hicache_metadata.py test/registered/unit/managers/test_hicache_controller_cp.py test/registered/unit/disaggregation/test_cp_shared_kv_transfer_mapping.py -q (91 passed)

Not-tested: Full CP shared KV + HiCache + EAGLE/MTP ETE server run after this commit

docs/advanced_features/nsa_prefill_cp_hicache_draft_kv_strong_sync_plan.md

@@ -35,6 +35,14 @@ eviction, or visibility decisions.
   zero-owned rank handling and frees the replay allocation on failure.
 - Phase 3 implemented: `HiRadixCache.reset()` clears target and draft host pools
   as one logical reset.
+- Phase 5 implemented: CP host-backed predicate now treats in-flight
+  write-through nodes as pending, not request-visible host hits; duplicate
+  write-through launch is suppressed while a node is in
+  `ongoing_write_through`.
+- Phase 6 implemented: controller tests now cover target+draft host eviction,
+  zero-owned draft metadata, and draft-before-target load readiness; CP draft
+  state-buffer registration is centralized and fails fast on target/draft state
+  mismatch when `SGLANG_CP_DRAFT_SHARED_KV=1`.
 - Adjacent controller invariant guard restored for CPU/fake-test tensors only:
   page-shaped host/device indices are validated in unit tests without
   reintroducing CUDA tensor truth-value checks or hot-path host sync.
@@ -574,6 +582,14 @@ the explicit-ready path before per-layer backup. Pending write is a valid
 lifecycle state, so it may return “not ready”; malformed metadata must still
 raise.
 
+Implemented decision: use the explicit-ready path. `_node_backuped()` still
+validates CP/draft metadata first, but returns `False` while the node id is in
+`ongoing_write_through`. This keeps in-flight D2H metadata out of request-visible
+host-hit admission until `writing_check()` observes the joint target+draft ack
+and removes the node from the pending table. `_inc_hit_count()` treats pending
+write-through as “already launched” and does not start a second write for the
+same node.
+
 ### Phase 6: Keep internal transfer split, but expose one logical operation
 
 Files:
@@ -615,6 +631,23 @@ Add controller tests:
    an error. It should not silently skip draft state buffers because that creates
    a target/draft cache-hit mismatch.
 
+Implemented status:
+
+- `evict_cp_host()` fails before freeing target host slots if draft HiCache is
+  attached but metadata lacks `draft_host_indices`; otherwise target and draft
+  host slots are freed as one logical operation.
+- `_write_cp()` zero-owned ranks keep explicit empty `draft_host_indices`, so
+  zero-owned target metadata is still draft-valid.
+- `start_loading()` is test-protected to submit draft H2D before completing the
+  corresponding target layer readiness event.
+- `append_cp_draft_state_buffers()` is the single helper used by prefill and
+  decode queue setup. It appends draft NSA state buffers to the target state
+  stream, preserves legacy skip behavior when draft shared KV is disabled, and
+  raises on target/draft state mismatch when draft shared KV is enabled.
+- Code-read confirmation: PD transfer page indices remain target-driven;
+  draft KV buffers are extra buffers behind the same page-index stream, not an
+  independent draft page-index stream.
+
 ### Phase 7: Remote validation
 
 Run targeted tests in the remote container because this repo's runtime deps and