Implement direction-independent calibration in peel subcommand

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This PR implements full "peeling" functionality in the peel subcommand by adding direction-independent (DI) calibration capabilities. Previously, the peel subcommand only performed ionospheric subtraction. Now it can perform complete peeling, which includes both ionospheric subtraction and DI calibration toward bright sources.

Key Changes

New CLI Arguments

• --peel N: Specify number of brightest sources to fully peel (DI calibrate)
• --di-max-iterations: Maximum DI calibration iterations (default: 50)
• --di-stop-threshold: DI calibration stop threshold (default: 1e-8)
• --di-min-threshold: DI calibration minimum threshold (default: 1e-4)

Parameter Validation

The hierarchy of source processing is now: --peel ≤ --iono-sub ≤ --num-sources

Implementation Details

• DI calibration runs on the final pass for sources within --peel count
• Uses existing calibrate_timeblock function from the di_calibrate module
• Both CPU and GPU implementations supported (GPU copies data to CPU for calibration)
• Provides convergence reporting and validation
• Maintains full backward compatibility with existing ionospheric-only peeling

Usage Examples

# Ionospheric subtraction only (existing behavior)
hyperdrive peel --iono-sub 10 [other args...]

# Full peeling with DI calibration on 3 brightest sources
hyperdrive peel --peel 3 --iono-sub 10 [other args...]

# With custom DI calibration parameters
hyperdrive peel --peel 3 --di-max-iterations 100 --di-stop-threshold 1e-10 [other args...]

What's the Difference?

Ionospheric Subtraction (--iono-sub):

• Solves for ionospheric refraction parameters α and β (∝ λ²)
• Applies scalar gain correction
• Subtracts ionosphere-corrected model from visibilities

Full Peeling (--peel):

• Performs ionospheric subtraction (as above)
• Additionally performs direction-independent calibration toward the source
• Solves for complex gain corrections per antenna and channel
• Corrects for scintillation and other phase/amplitude effects beyond the ionosphere

Current Limitations

The DI calibration solutions are computed and validated but not yet fully applied to the visibility data. This is marked as a TODO for future enhancement. The current implementation demonstrates the calibration convergence and provides the foundation for complete solution application.

Fixes #61.

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