Avoid index prefetch fallback noise when disabled

The missing index prefetcher state is expected when MLA/index prefetch is disabled. Logging it as an index-prefetch fallback made the normal synchronous materialize path look like a failed fast path and created noisy reports.

This keeps consume-miss warnings for enabled prefetchers, but suppresses missing-prefetcher warnings when the prefetch feature gate is off.

Constraint: Index prefetch is controlled by the same MLA prefetch enable gate in the current pipeline.
Rejected: Suppress all index prefetch fallback warnings | consume misses still indicate an enabled prefetch pipeline failed to serve a requested buffer.
Confidence: high
Scope-risk: narrow
Directive: Missing prefetcher is not a fallback when SGLANG_CP_SHARED_KV_ENABLE_MLA_PREFETCH=0; do not re-add warning noise without a separate enable gate.
Tested: Local py_compile for nsa_indexer.py and test_cp_shared_kv_runtime.py.
Tested: Remote g0034 targeted regression set passed 4 tests: disabled sync materialize no warning, consume miss warning, first-layer no-warning, enabled create-skip warning.
Not-tested: Full CUDA ETE run with logs.

docs/advanced_features/nsa_prefill_cp_page_aligned_cache_contract.md

@@ -5462,3 +5462,30 @@ Remaining risk:
   requires higher-level eviction/allocation policy to preserve larger free runs;
   the allocator now avoids the worst CPU scan cost but cannot create physical
   contiguity that the free set does not contain.
+
+### C127 — 2026-06-02 Disabled index prefetch must not emit fallback warnings
+
+Finding:
+
+- `CpSharedKVIndexPrefetcher.maybe_create()` correctly returns `None` without a
+  warning when `SGLANG_CP_SHARED_KV_ENABLE_MLA_PREFETCH=0`.
+- The later synchronous materialization path in `nsa_indexer.py` still logged
+  `[CP_SHARED_KV_FALLBACK][index_prefetch] reason=missing_prefetcher` whenever
+  `forward_batch.cp_shared_kv_index_prefetcher` was `None`.
+- With MLA/index prefetch disabled, `None` is the expected state, so this warning
+  made normal sync materialize look like a failed fast path and spammed logs.
+
+Correction:
+
+- Gate only the `missing_prefetcher` / `current_missing_prefetcher` warnings on
+  `cp_shared_kv_mla_prefetch_enabled()`.
+- Keep `consume_miss` warnings unchanged: if a prefetcher exists but cannot serve
+  the requested layer/current buffer, that is still a real enabled-prefetch miss.
+
+Validation:
+
+- Remote RED/GREEN test added:
+  `test_index_sync_materialize_does_not_warn_when_prefetch_disabled`.
+- Remote targeted regression set passed (`4 passed`): disabled sync materialize,
+  consume miss warning, first-layer no-warning, and create-skip warning when
+  enabled.