Fail fast when CP MLA partial-current compose is unavailable

Mixed prefix/current MLA reuse depends on the page-slot prefetch compose path to preserve padded tail semantics. The compact materialize/current merge path can re-expose suffix slack as valid dense rows, so the backend now raises with an explicit fail-fast marker instead of silently falling back.

Constraint: Page-aligned CP shared KV contract requires suffix tail slack to remain invalid.

Rejected: Keep compact merge as a fallback | it has a different dense-row contract and can hide correctness bugs.

Confidence: high

Scope-risk: moderate

Directive: Do not reintroduce merge_materialized_and_current_kv on MLA partial-current reuse without a page-slot correctness proof.

Tested: Remote py_compile for nsa_backend.py and test_nsa_cp_utils.py in g0034 container.

Tested: Remote pytest test_nsa_cp_utils.py::TestNSAInSeqCPUtils::test_mla_partial_current_path_fails_fast_instead_of_compact_fallback.

Tested: Remote pytest test_nsa_cp_utils.py test_cp_shared_kv_layout.py test_cp_shared_kv_runtime.py: 124 passed, 5 warnings.

Not-tested: Live ETE traffic and CUDA kernel execution for this fail-fast path.

Co-authored-by: OmX <omx@oh-my-codex.dev>

docs/advanced_features/nsa_prefill_cp_page_aligned_cache_contract.md

@@ -115,8 +115,8 @@ slot-dense locs but no valid current KV row.
 
 ## Root-cause record: unsafe partial-current prefetch
 
-The current MLA partial-current prefetch path has a narrower contract than the
-page-level metadata it consumes.
+The original MLA partial-current prefetch path had a narrower contract than the
+page-level metadata it consumed.
 
 Evidence:
 
@@ -126,8 +126,8 @@ Evidence:
 - `cp_shared_kv_runtime.py::merge_materialized_and_current_kv()` only remaps
   entries present in `current_locs`; all non-current entries keep the existing
   `materialized_dense_locs`.
-- `nsa_backend.py` calls `consume_prefix_with_current()` whenever an MLA
-  prefetcher exists on a mixed prefix/current batch, before falling back to the
+- `nsa_backend.py` called `consume_prefix_with_current()` whenever an MLA
+  prefetcher existed on a mixed prefix/current batch, before falling back to the
   older partial materialize path.
 
 That is safe only when the page-level suffix represented by the page table is
@@ -137,13 +137,18 @@ exactly covered by `current_locs`.  It is not safe when:
 ceil(extend_len / page_size) * page_size > extend_len
 ```
 
-The older non-prefetch partial path is safer because it materializes all
-non-current logical locations and only substitutes rows that match
-`current_locs`.
+The older non-prefetch partial path looked safer because it materialized all
+non-current logical locations and only substituted rows that matched
+`current_locs`, but it still re-entered the compact
+`merge_materialized_and_current_kv()` contract.  Under the page-aligned cache
+contract this is no longer an acceptable fallback for mixed prefix/current MLA
+reuse.
 
-The fix should not be a permanent fallback.  The intended fix is to make the
-prefetch compose path create a page-padded current suffix buffer and map suffix
-slack to `-1`.
+The fix is intentionally not a permanent fallback.  The page-slot prefetch
+compose path creates a page-padded current suffix buffer and maps suffix slack
+to `-1`.  If that compose path is unavailable, the runtime now fails fast with
+`[CP_SHARED_KV_FAIL_FAST][mla_partial_current_prefetch]` instead of silently
+using compact materialize/current merge.
 
 ## Existing evidence in the codebase
 
@@ -445,7 +450,7 @@ Tests:
 - `extend_len=3 pages, cp_size=8` should produce three compute owners, not
   eight or sixteen.
 
-### C4. MLA partial-current prefetch compose is partially corrected
+### C4. MLA partial-current prefetch compose is fail-fast without prefetch
 
 Current state:
 
@@ -455,16 +460,19 @@ Current state:
   their dense page slots and masks non-current locs in current pages to `-1`.
 - A TAI primitive exists for the CUDA hot path, with a visible warning fallback
   to the Torch reference when unavailable.
+- `nsa_backend.py` no longer uses the compact
+  `merge_materialized_and_current_kv()` helper when MLA partial-current reuse
+  has prefix + current suffix but page-slot prefetch compose is unavailable.
+  That path now raises
+  `[CP_SHARED_KV_FAIL_FAST][mla_partial_current_prefetch]` with layer/rank,
+  prefix/extend lengths, table shape, and remap capacity metadata.
 
-Correction still needed:
+Design decision:
 
-- Verify that every MLA mixed prefix/current path uses the page-slot compose
-  helper when a dense slot buffer already exists.
-- Audit the non-prefetch fallback in `nsa_backend.py`: it still calls
-  `merge_materialized_and_current_kv()`, which is the compact-current helper.
-  That path may be correct only when the materialized loc table has already
-  masked suffix slack.  This must be proven by tests or changed to the same
-  page-slot helper.
+- Mixed prefix/current MLA reuse must use the page-slot compose path.  If the
+  prefetcher is missing or `consume_prefix_with_current()` cannot provide a
+  composed buffer, failing immediately is safer than re-entering the compact
+  materialize/current merge path.
 - Keep `merge_materialized_and_current_kv()` documented as compact-materialize
   only; do not silently use it for page-slot prefetch compose.
 
@@ -472,8 +480,9 @@ Tests:
 
 - Existing MLA slack test covers a tail page where only part of the suffix page
   is valid.
-- Add a non-prefetch partial-current test with the same tail-page shape before
-  changing the fallback path.
+- `test_mla_partial_current_path_fails_fast_instead_of_compact_fallback`
+  statically guards `nsa_backend.py` against reintroducing
+  `merge_materialized_and_current_kv()` calls and requires the fail-fast marker.
 - CUDA execution of the TAI primitive must be verified remotely; do not test
   CUDA locally.
 
@@ -1014,15 +1023,17 @@ Completed C14:
   123 passed, 5 warnings
   ```
 
-## Testing requirements
+## Testing requirements / status
 
-Add targeted tests before implementation:
+Targeted coverage needed for this contract:
 
 1. `extend_len % page_size != 0` MLA partial current reuse:
    - prefix page-aligned,
    - suffix has page slack,
    - dense buffer includes padded suffix rows,
    - slack locs remap to `-1`.
+   - status: page-slot compose is covered; non-prefetch compact fallback is
+     guarded by fail-fast source test until a direct forward-path fixture exists.
 
 2. Equivalent NSA index partial reuse case:
    - tail page included in page table,