Refactor: Split _conversion.py into domain modules, improve naming in _helper.py

[Wolfvin]

Structural Refactoring with Regression Testing Proof

All changes are backward-compatible.

What was refactored

1. _conversion.py (444 lines) split into _conversion/ package

   • _decibels.py (141 lines): db(), linear()
   • _snr.py (315 lines): All 6 SNR conversion functions
   • init.py re-exports everything

2. _helper.py naming improvements

   • _argument_names() -> _extract_caller_argument_names()
   • verify_scalar() -> verify_and_validate_scalar()
   • verify_arraylike() -> verify_and_validate_arraylike()
   • convert_output() -> normalize_output_type()
   • convert_to_scalar() -> demote_numpy_scalar_to_native()
   • All old names kept as backward-compatible aliases

3. _signal.py docstring improvements

Regression Testing Proof (Regrets)

18 clusters tested, 4 chains validated, 0 false positives.

All 4 verifications passed:

1. All 18 Regrets clusters GREEN
2. Direct output matches KEBENARAN 1 (raw ground truth)
3. Fingerprints match KEBENARAN 2 (fingerprint ground truth)
4. All 4 chain hashes match (multi-step flow validation)

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.31298% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.20%. Comparing base (8b8b0b3) to head (4b6e39c).
⚠️ Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sdr/_helper.py	83.63%	9 Missing ⚠️
src/sdr/_signal.py	81.81%	4 Missing ⚠️
src/sdr/_conversion/_decibels.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #489      +/-   ##
==========================================
+ Coverage   81.81%   82.20%   +0.38%     
==========================================
  Files          63       65       +2     
  Lines        3960     4119     +159     
==========================================
+ Hits         3240     3386     +146     
- Misses        720      733      +13     

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