feat: mechanical mobility and the FRF family (ISO 7626-1:2011)
Add the ISO 7626-1:2011 frequency-response-function family — the
motion-per-force FRFs (receptance, mobility, accelerance) and their
force-per-motion reciprocals (dynamic stiffness, impedance, apparent
mass) of Table 1 — plus the single-degree-of-freedom reference resonator
of Annex A that underpins the structure-borne source and transmission
standards (ISO 9611, ISO 10846, EN 15657, EN 12354-5).
New module `mechanical_mobility`:
- `convert_frf` converts between any two of the six FRFs, pivoting
through the receptance H (Y = jωH, A = −ω²H, reciprocals = 1/FRF).
- `sdof_receptance` / `sdof_mobility` / `sdof_accelerance` and
`resonance_frequency` for the viscously damped SDOF resonator
H = 1/(k − ω²m + jωc).
- `MobilityResult` (`.magnitude`, `.phase`, `.to(target)`, `.plot()`)
and `sdof_mobility_result`.
Conformance is anchored on the closed-form SDOF identities: the
driving-point mobility peak |Y(f0)| = 1/c, the static receptance
H(0) = 1/k, and the exact Table-1 reciprocity Z·Y = 1 (126/126).
Adds the module figure, the EN/ES guides, the docs index and
api-reference rows, and the sidebar section.