feat: dynamic transfer stiffness of resilient elements (ISO 10846)

Add the ISO 10846 dynamic transfer stiffness k2,1 = F2,b/u1 of resilient
elements (vibration isolators, mounts, bellows, hoses) — the quantity
that characterises their vibro-acoustic transmission (Part 1, clause 5).
This is the third PR of the structural vibroacoustics series (#16) and
feeds ISO 9611, EN 15657 and EN 12354-5.

New module `transfer_stiffness`:
- `transfer_stiffness_level` Lk = 20 lg(|k2,1|/k0) re k0 = 1 N/m
(Parts 2/3, 3.17) and `loss_factor` eta = Im/Re (Part 1, 3.8).
- `transfer_stiffness_direct` k2,1 = F2,b/u1 (direct method, Part 2).
- `transfer_stiffness_indirect` k2,1 = -(2 pi f)^2 (m2 + mf) T from the
vibration transmissibility and a blocking mass (indirect method,
Part 3, Formula 1), plus `base_transmissibility` for the ideal
mass-loaded Kelvin-Voigt element used in examples and the figure.
- `TransferStiffnessResult` (`.level`, `.loss_factor`, `.magnitude`,
`.to("impedance"/"apparent_mass")`, `.plot()`) and
`indirect_transfer_stiffness_result`.

The dynamic stiffness is the reciprocal receptance of the FRF family, so
k = jw Z = -w^2 m_eff reuse the mechanical-mobility convert_frf pivot
(Part 1, Annex A / Table A.2).

Conformance is anchored on the standard's closed-form definitions: the
level of a decade of stiffness (120 dB re 1 N/m), the indirect inertia
relation, and the Table-A.2 identity k = jw Z (129/129).

Adds the module figure, the EN/ES guides, the docs index and
api-reference rows, and the sidebar entry.