Persist sim options before StartSimul so the seed is honored (AB#2742)

run_simulation(seed=N) packed the seed correctly ([SeedFixed]:1 /
[seed0]:N, matching ModelRisk's PackToStringList), and the array
marshalled fine — verified by reading back ModelRisk's own parsed
SimOpt_SeedFixed=1 / SimOpt_Seed0=N after feeding the exact payload to
VoseSetSimulOptions12. But the simulation still ran non-reproducibly.

Root cause: ModelRisk's per-cell-twister Manual-Seed mode sources its
base seed from the WORKBOOK-persisted SimOpt_SeedFixed / SimOpt_Seed0
defined-names (written by SaveOptionsToWorkbook), NOT from the per-call
options passed to VoseStartSimulCustom12. The bridge set the per-call
[seed0] (ignored by the twister) but never persisted it to the workbook,
so Manual Seed stayed OFF -> Random mode -> different stream every run.

Fix: call VoseSetSimulOptions12 with the same options payload before
VoseStartSimulCustom12. That runs SaveOptionsToWorkbook, persisting
SimOpt_SeedFixed=1 / SimOpt_Seed0=N, which the twister then honors.

Verified end-to-end against the engine oracle (single-cell VoseUniform):
- two runs at seed=12345 -> byte-identical streams (was: differing)
- seed=12345 vs seed=555 -> different reproducible streams (seed now drives it)
- the seed=12345 stream matches mrengine's per-cell twister-0 reference
  (0.929616, 0.890155, 0.316376, 0.130707, 0.183919, 0.039759, ...) exactly,
  validating the byte-identical parity path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vosesoft

src/modelrisk_mcp/bridge/simulation.py

@@ -124,6 +124,14 @@ class SimulationRunResult:
 # "Vose" + X (ModelRiskCloude/CLAUDE.md:63).
 _CMD_START_SIM = "VoseStartSimulCustom12"
 _CMD_GET_DATA_SZ = "VoseGetDataSZ12"
+# Persists the simulation options to the workbook as SimOpt_* defined-names via
+# SaveOptionsToWorkbook (SimulationCommonOptionsReadWrite.cpp). REQUIRED for the
+# seed: ModelRisk's per-cell-twister Manual-Seed mode reads its base seed from the
+# workbook-persisted SimOpt_SeedFixed / SimOpt_Seed0 names, NOT from the per-call
+# options handed to VoseStartSimulCustom12. Without this call the per-call
+# [SeedFixed]/[seed0] are parsed but ignored by the twister -> Random mode ->
+# non-reproducible streams (AB#2742; verified end-to-end against the engine oracle).
+_CMD_SET_SIM_OPTS = "VoseSetSimulOptions12"
 
 # Operation prefix the SimulationObj_VBA dispatcher matches on for the
 # save path. PackSessionName format: "h<hwnd>_<Operation>_<book_name>"
@@ -346,6 +354,10 @@ def _invoke_start_simulation(self, opts: SimulationOptions) -> None:
         self._ensure_xll_registered()
         try:
             options_2d = [opts.to_string_list()]  # 1 row x N cols
+            # Persist options (esp. SeedFixed/seed0) to the workbook FIRST so the
+            # per-cell-twister Manual-Seed engine actually honors the seed. See
+            # _CMD_SET_SIM_OPTS above for the full rationale (AB#2742).
+            app.api.Run(_CMD_SET_SIM_OPTS, options_2d)
             app.api.Run(_CMD_START_SIM, options_2d)
         except Exception as exc:
             raise SimulationFailedError(