Add extend_single_year_dataset for fast dataset year projection

[anth-volk]

Fixes #7699

Summary

• Adds USSingleYearDataset and USMultiYearDataset schema classes (modeled after UK equivalents) for entity-level dataset handling
• Adds extend_single_year_dataset() with multiplicative uprating derived from system.variables[var].uprating at runtime — no separate uprating list to maintain
• Adds dual-path loading in Microsimulation.__init__: auto-detects entity-level HDFStore files, loads them as USSingleYearDataset, extends via uprating, and passes the multi-year result to policyengine-core
• Legacy h5py variable-centric files continue to work via the existing code path

Depends on

• Add entity-level HDFStore output format alongside h5py policyengine-us-data#568 (HDFStore output format from -us-data)

Test plan

• Load an HDFStore file: Microsimulation(dataset="path/to/STATE.hdfstore.h5") — verify it loads and extends correctly
• Load a legacy h5py file: Microsimulation(dataset="path/to/STATE.h5") — verify existing path still works
• Verify uprated variables (e.g. employment_income) grow year-over-year
• Verify non-uprated variables (e.g. age) are carried forward unchanged
• Verify _is_hdfstore_format() correctly distinguishes the two file formats

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