Default to single sector and enable heterogeneous CES epsilon

[vahid-ahmadi]

Summary

• Default to M=1 (single sector): The 8-sector industry calibration is now opt-in via multi_sector=True on solve_steady_state() / run_transition_path(), or multi-sector CLI flag. This makes the default run faster and matches OG-Core's built-in M=1 defaults.
• Enable heterogeneous CES epsilon in multi-sector mode: Uses calibrated literature values (Chirinko 2008, Knoblach et al. 2020) ranging from 0.40 (Real Estate) to 1.30 (Business Services), instead of forcing all sectors to Cobb-Douglas (ε=1.0).
• Recalibrate TFP (Z) for CES: Solow residuals now use the CES production function when ε≠1, ensuring Z values are consistent with the production technology.

Usage

# Single sector (default, fast)
uv run python examples/run_oguk.py ss pooled
uv run python examples/run_oguk.py tpi

# 8-sector with heterogeneous CES
uv run python examples/run_oguk.py ss pooled multi-sector
uv run python examples/run_oguk.py tpi multi-sector

Dependencies

Requires PSLmodels/OG-Core#1096 (numerical guards for CES production functions) to prevent NaN when ε<1 and inputs are near zero.

Test plan

• 8-sector SS with heterogeneous epsilon converges (resource constraint ~1e-6)
• Single-sector SS/TPI produces same results as before this PR
• 8-sector TPI with heterogeneous epsilon converges (requires OG-Core#1096)

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