Clarify and unify add_projections semantics for SONATA circuits

[ilkilic]

add_projections previously had ambiguous semantics and mixed node- and
edge-level concepts. This PR clarifies the behavior by defining projection
handling strictly in terms of SONATA edge populations.

Changes & compatibility

• Projection selection is now edge-population–based (SONATA edges),
  not node populations.

• Clarified semantics of add_projections (SONATA):

  • False (default): intrinsic connectivity only
  • None: treated as False for backward compatibility
  • True: intrinsic connectivity plus all projection
    (virtual-source) edge populations
  • str / list[str]: intrinsic connectivity plus the specified
    identifiers (edge population names, with limited legacy support for
    source node population names)

• get_all_projection_names() now returns edge population names.

• Tests and docstrings have been updated accordingly.

Backward compatibility

This PR intentionally changes behavior to remove ambiguity:

• add_projections=True changes semantics

  • Before: projection / virtual-source connections only
  • Now: intrinsic connectivity and all projection edge populations

• add_projections=False now consistently means intrinsic connectivity only.

• add_projections=None is supported as a legacy alias for False.

• add_projections=str / list[str] remains supported, but identifiers
  now primarily refer to edge population names.

This change aligns Bluecellulab more closely with Neurodamus semantics by
making intrinsic connectivity and projections explicit, without reproducing
Neurodamus-specific heuristics.

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bluecellulab/circuit_simulation.py	75.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
...ab/circuit/circuit_access/sonata_circuit_access.py	98.79% <100.00%> (+0.93%)	⬆️
...ellulab/circuit/config/sonata_simulation_config.py	97.86% <100.00%> (-0.02%)	⬇️
tests/test_cell/test_core.py	99.40% <ø> (ø)
tests/test_circuit/test_circuit_access.py	100.00% <100.00%> (ø)
bluecellulab/circuit_simulation.py	86.66% <75.00%> (+0.07%)	⬆️

... and 2 files with indirect coverage changes

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