Add Impedance boundary condition

[ncdorn]

Current situation

This branch adds support for an IMPEDANCE outlet boundary condition based on an Olufsen et al. (1999) time-domain convolution. The need for this BC is to represent the frequency-dependent downstream response of a structured tree, including wave reflection and distributed vascular memory, which cannot be captured well by a simple resistance or RCR model.

The implementation treats outlet pressure as

P = Pd + z0 * Q + sum(z[m] * Q_lag[m])

where z is a one-cycle discrete impedance kernel and the lagged flow terms come from accepted solution history. To make that work in svZeroDSolver, the solver needed explicit support for boundary-condition memory that persists across accepted time steps, can be restored for coupled trial steps, and stays consistent with the configured cardiac-period discretization.

Release Notes

Add a bullet point list summary of the feature and possible migration guides if this is a breaking change so this section can be added to the release notes.

• Added a new IMPEDANCE boundary condition for structured-tree style outlet loading using a periodic discrete convolution kernel.
• Added support for bc_values.z, optional Pd, and convolution controls via convolution_mode = exact|truncated and num_kernel_terms.
• Added solver/model hooks for persistent per-block state so convolution history is only advanced on accepted time steps.
• Added coupling-safe rollback/commit behavior in the interface so repeated trial solves start from the same accepted impedance history.
• Added validation that the impedance kernel length matches one cardiac cycle in time steps and that coupled runs use the required time-step layout.
• Added regression and interface tests covering the new BC and its persistent-memory behavior.

Documentation

Documentation is included in the README with:

• the IMPEDANCE BC definition,
• required and optional JSON fields,
• timing/discretization requirements,
• coupled vs non-coupled setup expectations,
• the runtime implication of truncated convolution.

Testing

Coverage added in this branch includes:

• a new solver regression case: tests/cases/pulsatileFlow_R_impedance.json with reference output,
• interface test coverage for trial/accept semantics and persistent-state commits for IMPEDANCE,
• updates to the Python solver test list so the new case runs with the standard regression suite.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

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