feat(decoders): ✨ add public decode_masked_flag helper

[svasek]

🧩 What

Add a public decode_masked_flag(flag, data) helper to neopool_modbus.decoders.

It isolates a MaskedFlag's field from an async_read_all() snapshot via the library's own mask/shift layout, so callers do not have to maintain protocol masks themselves. Returns None when the shared register is absent.

💡 Why

async_set_masked_register already owns the write side of packed registers (for example HIDRO_COVER_REDUCTION_PERCENT and HIDRO_SHUTDOWN_TEMPERATURE, which share register 0x042D). There was no matching read helper, so consumers had to duplicate the same masks and shifts to decode those fields. This adds the read counterpart and keeps the mask layout a single library concern.

🔧 Changes

• ✨ decoders.py: new decode_masked_flag(flag, data) reading _MASKED_FLAG_LAYOUT
• 📦 export it in decoders.__all__
• ✅ test_decoders.py: cover both fields from a packed 0x0C19 (25 / 12) and the None-when-absent path
• 📝 README.md: list the helper in the public API and document it as the read counterpart to async_set_masked_register

✅ Verification

• pytest full suite passes; decoders.py at 100% coverage
• ruff check, ruff format --check, basedpyright clean on the touched files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f879d54) to head (edfcb0d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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##              main       #66   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines         1223      1230    +7     
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+ Hits          1223      1230    +7     

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