feat: add structured logging, CSV metadata headers, and CLI improvements

- New module: beta_spectrum/logging_utils.py
  - LoggingConfig dataclass with level, log_file, format options
  - setup_logging() one-shot configuration (opt-in for library use)
  - get_logger() for named child loggers
  - get_git_short_hash() for commit identification
  - _find_git_root() for repo detection

- CLI enhancements (beta_spectrum/cli.py):
  - --verbose/-v (repeatable): -v=INFO, -vv=DEBUG
  - --quiet/-q: suppress all terminal output
  - --log-file PATH: append log to file
  - --dry-run: validate input, display resolved config, exit
  - --version: show package version
  - Replace print() with structured logging throughout
  - Handle KeyboardInterrupt gracefully

- CSV metadata headers (BetaSpectrumAnalyzer.export_to_csv):
  - YAML-style # comments with version, timestamp, source, nuclide info
  - Corrections enabled, e_step, detector settings, git commit hash

- Base class integration (SpectrumComponent):
  - Optional logger parameter for debug/info output (opt-in)

- BetaSpectrum integration:
  - Optional logger parameter passed through from_config()
  - Detector logging in create_detector_from_config()
  - Convolution logging in convolve_with_detector()

- Tests: tests/test_logging_utils.py (28 tests)
  - LoggingConfig, setup_logging, get_logger, git hash, CSV headers, CLI flags

- Update TODO.md: mark B8 as complete, update Current Status

beta_spectrum/base.py

@@ -1,6 +1,10 @@
 # base.py
 from __future__ import annotations
+
+import logging
 from abc import ABC, abstractmethod
+from typing import Optional
+
 import numpy as np
 
 ArrayLike = np.ndarray
@@ -8,12 +12,21 @@ ArrayLike = np.ndarray
 
 class SpectrumComponent(ABC):
     """
-    Base class for multiplicative components of beta-spectrum
+    Base class for multiplicative components of beta-spectrum.
+
+    Parameters
+    ----------
+    logger : logging.Logger, optional
+        Logger instance for debug/info output. If None, logging is disabled.
+        Opt-in design: components are silent by default for library use.
     """
 
+    def __init__(self, logger: Optional["logging.Logger"] = None) -> None:
+        self._logger = logger
+
     @abstractmethod
     def __call__(self, W: ArrayLike) -> ArrayLike:
         """
-        Evaluate component  at total energy W (in m_e units)
+        Evaluate component at total energy W (in m_e units).
         """
         pass