fix(heldout): declare assignment-design trial count

[seonghobae]

Stack

Draft successor of #1097 (525991db). Parents #1096, #1095, #1093, #1092, #1091, #1090, #1074, and psychometric KPI owner #1067 stay open. This PR does not close #1097.

Exact head: e39a99f0ecd1e331f904f03df6a546559d9924eb.

Problem

Held-out inverse-propensity assignment evidence hid ASSIGNMENT_TRIALS = 24_000, so logging-design RMSE used a repository-selected Monte Carlo sample size.

Change

• Require declared trial_count on _validate_assignment_design. Missing, boolean, or non-positive values fail closed.
• The harness run still writes 24,000 as this run's choice and records trials.
• ADR 0049 is Proposed. Production route/conduct defaults stay locked.

Verification

Local smallest tests:

• 60 passed in 15.74s (tests/test_psychometric_routing.py, tests/test_psychometric_benchmark_boundaries.py, tests/test_planning_adr_identifiers.py)
• interrogate on the held-out script: 100%

This is declaration-contract evidence, not buyer-held-out accuracy, p95 latency, or protected merge.

Remaining no-heuristics work

Other harness sample sizes (DIF, reliability, exploration rate) remain later slices. No production policy or release is authorized. Parent #1067 still needs independent review.

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[seonghobae]

Normal predecessor integration completed at ae5aabf, preserving original assignment-trial delta e39a99f and incorporating predecessor 0771905. Includes the no-detection censoring repair, separate false-alarm/detection denominators, null candidate when no calibration threshold is eligible, and retained calibration results. Merged-tree validation: 27 tests passed in 29.96 seconds (boundary suite and ADR identifier suite), Ruff passed, diff whitespace check passed. CodeGraph indexing completed. No force push or discarded delta. Prior-head hosted checks are not evidence for this new head; protected checks/reviews and release remain pending.