Clean IMF/OECD fallback handling

[agurgone]

Reopens the useful part of #75 as a clean PR without the unrelated branch history.

Changes:

• Keep IMF national-accounts growth output limited to series actually present in IMF data instead of filling missing components from GDP.
• Let merged exogenous national accounts use OECD component growth when IMF lacks those components, with GDP fallback only where explicitly requested.
• Keep nominal and real household-consumption-to-GDP ratios aligned.
• Add focused regression coverage for the IMF/OECD fallback behavior.

Validation:

• UV_CACHE_DIR=/private/tmp/uv-cache uv run --frozen --with ruff ruff format macro_data/readers/economic_data/imf_reader.py macro_data/readers/economic_data/oecd_economic_data.py macro_data/readers/exogenous_data.py tests/test_macro_data/unit/test_readers/test_economic_data/test_imf_reader.py tests/test_macro_data/unit/test_readers/test_exogenous.py
• UV_CACHE_DIR=/private/tmp/uv-cache uv run --with pytest pytest tests/test_macro_data/unit/test_readers/test_economic_data/test_imf_reader.py tests/test_macro_data/unit/test_readers/test_exogenous.py