Add ScaleUpRunner harness for synthesizer scale-up benchmark

Implements the stage-1/2/3 protocol from docs/synthesizer-benchmark-scale-up.md
as a real runnable harness.

Components:
  - src/microplex_us/bakeoff/scale_up.py
      * ScaleUpStageConfig: frozen dataclass with curated 50-column default
        (14 demographics + 36 income/wealth/benefit targets)
      * ScaleUpRunner: load_frame, split, fit_and_generate, run
      * _load_enhanced_cps: entity-aware loader that broadcasts
        household / SPM-unit / tax-unit / family / marital-unit variables
        down to person level via person_<entity>_id -> <entity>_id lookups
      * Per-method metrics: PRDC precision/density/coverage (via prdc
        library), wall time, peak RSS, rare-cell preservation ratios
        (elderly self-employed, young dividend, disabled SSDI,
        top-1 % employment), zero-rate MAE
      * CLI: python -m microplex_us.bakeoff.scale_up --stage stage1 ...
      * Stage configs: stage1 (~77k from ECPS), stage2 (1M, needs larger
        source), stage3 (v6 seed-ready 3.4M x 155)

  - tests/bakeoff/test_scale_up.py
      * Smoke tests on a 500-row, 5-column, ZI-QRF-only slice
      * Entity-broadcast verification via real ECPS loading
      * Column-missing error path
      * Default column-set sanity check

Notable limitations recorded for follow-up:
  - state_fips / snap_reported / net_worth / housing_assistance and other
    non-person entity variables are now correctly broadcast to person
    level via ID lookup. This was the blocker for a flat DataFrame.
  - enhanced_cps_2024 has 77k persons, not the 100k stage-1 target.
    n_rows=None now uses all available.
  - is_household_head is not in ECPS; replaced with is_separated.

Not in this commit (deliberate):
  - No execution of stage1 / stage2 / stage3 runs yet
  - No CTGAN / TVAE support (present in registry, not in default method set)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaxGhenis

src/microplex_us/bakeoff/__init__.py

@@ -0,0 +1,43 @@
+"""Scale-up benchmark harness for synthesizer comparison.
+
+Implements the stage-1/2/3 scale-up protocol from
+`docs/synthesizer-benchmark-scale-up.md`: load real enhanced_cps_2024,
+sub-sample to the stage's row count, fit each specified synthesizer on the
+conditioning + target column set, and report PRDC coverage, training wall
+time, peak RSS, and rare-cell preservation.
+
+Use from the CLI:
+
+    uv run python -m microplex_us.bakeoff.scale_up \\
+        --stage stage1 \\
+        --methods ZI-QRF ZI-MAF ZI-QDNN \\
+        --output artifacts/scale_up_stage1.json
+
+or programmatically:
+
+    from microplex_us.bakeoff import ScaleUpRunner, stage1_config
+    runner = ScaleUpRunner(stage1_config())
+    results = runner.run()
+"""
+
+from microplex_us.bakeoff.scale_up import (
+    ScaleUpResult,
+    ScaleUpRunner,
+    ScaleUpStageConfig,
+    DEFAULT_CONDITION_COLS,
+    DEFAULT_TARGET_COLS,
+    stage1_config,
+    stage2_config,
+    stage3_config,
+)
+
+__all__ = [
+    "ScaleUpResult",
+    "ScaleUpRunner",
+    "ScaleUpStageConfig",
+    "DEFAULT_CONDITION_COLS",
+    "DEFAULT_TARGET_COLS",
+    "stage1_config",
+    "stage2_config",
+    "stage3_config",
+]