moe placement: adopt the writer's real table as a checked fixture, not prose (#1702)

CMakeLists.txt

@@ -588,7 +588,11 @@ add_dependencies(onebit spec_decode_test)
 add_executable(moe_placement_test tests/moe_placement_test.cpp)
 target_include_directories(moe_placement_test PRIVATE app)
 target_link_libraries(moe_placement_test PRIVATE nlohmann_json::nlohmann_json)
+# engine#1702: the second arm reads the writer's real output for the Flash-Next decode trace, so the fixture the issue
+# quotes (48 layers / 512 experts / 2 groups, held-out share 0.1892 over 5280 steps) is checked by the reader, not by prose.
 add_test(NAME moe_placement_reader COMMAND moe_placement_test)
+add_test(NAME moe_placement_reader_fixture COMMAND moe_placement_test
+         ${CMAKE_SOURCE_DIR}/tests/fixtures/moe_placement/flash-next-decode.placement.json)
 add_dependencies(onebit moe_placement_test)
 # The NPU route's bounded load retry (app/npu_load_retry.h, engine #1010, docs/npu.md "When a load follows an evict"):
 # a load started right after an evict can hit the previous backend's hardware context still being torn down, which used

tests/fixtures/moe_placement/README.md

@@ -0,0 +1,60 @@
+<!--
+Copyright 2026 bong-water-water-bong
+SPDX-License-Identifier: Apache-2.0
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# moe_placement fixtures
+
+`flash-next-decode.placement.json` is the **real output of the shipped writer** for a real trace, kept
+here so the reader is tested against something the pipeline actually produces rather than only against
+tables built by hand in the test.
+
+## What it is
+
+Qwen3.8-Flash-Next (48 routed layers, 512 experts, top-10) decode trace, 2 device groups:
+
+```
+$ python3 tools/moe_placement.py <trace> -g 2 -e 512 -o flash-next-decode.placement.json
+moe_placement: 48 layer(s), 2 group(s); one-device share 52.3% over 10560 decode steps IN-SAMPLE
+               (contiguous control 0.2%); held out 18.9% over 5280 steps
+```
+
+| property | value |
+|---|---|
+| schema | `1bit/moe-placement/1` |
+| groups | 2 |
+| layers | 48, keyed by decimal layer number |
+| per layer | every expert id `0..511` exactly once (48 of 48 layers are permutations) |
+| `share_in_sample` | 0.523 |
+| `share_held_out` | **0.1892** (above the reader's 15 % floor) |
+| `held_out_steps` | 5280 |
+| per-layer `unseen` | 138 of 512 experts never fire in a 220-step trace — informational; unseen experts are still assigned a group, which is why the permutation rule holds |
+
+## Why it is a fixture and not a shipped placement
+
+The numbers are a property of **this trace**, not of the model: a different prompt mix moves the
+one-device share (the issue's table shows 44–60 % across three traces and two models). So this is a
+regression fixture for the writer→reader path, not a recommendation to place a model this way.
+
+## Regenerating
+
+The writer is deterministic for a given trace and seed; regenerate with the command above. A new fixture
+must keep `share_held_out >= 0.15` or the reader refuses it, which is the check the fixture exists to
+exercise.
+
+## See also
+
+- `tools/moe_placement.py` — the writer (CI-registered via `add_python_test(moe_placement … --self-test)`).
+- `app/moe_placement.h` — the reader, and the only authority on whether a table is usable.
+- `docs/moe-streaming.md` — what consumes a table, and why the two-engine splitter is the next step.