feat: installed structure-borne sound from equipment (EN 12354-5) (#140)

Add the EN 12354-5:2009 prediction of the receiving-room sound pressure
level from building service equipment that injects structure-borne sound
into the building. Final PR of the structural vibroacoustics series (#16),
closing the chain source (EN 15657) -> coupling (ISO 10846 / ISO 7626
mobilities) -> building transmission.

New module `installed_structure_borne`:
- `coupling_term` DC = 10 lg(|Ys+Yi+Yk|^2/(|Ys| Re{Yi})) (Formula 19b/e),
with force-source (19c) and velocity-source (19d) limits.
- `installed_structure_borne_power_level` LWs,inst = LWs,c - DC (18b).
- `structure_borne_pressure_level_path` Ln,s,ij = LWs,inst - Dsa - Rij,ref
- 10 lg(Si/S0) - 10 lg(A0/4) (18a, S0 = A0 = 10 m2) and
`total_structure_borne_pressure_level` energetic path sum (17).
- `InstalledSourceResult` (`.overall_level`, `.plot()`) and
`installed_source_prediction` for a multi-path prediction.

The source and receiver mobilities/impedances are those of
mechanical_mobility and transfer_stiffness; LWs,c comes from EN 15657.

Conformance is anchored on the coupling-term force-source limit, the
installed-power identity and the area/absorption terms of Formula 18a;
Dsa and Rij,ref are inputs (measurement / EN 12354-1 / Annexes D, F)
(137/137).

Adds the cascade figure (characteristic -> installed -> paths -> total),
the EN/ES guides, the docs index and api-reference rows, and the sidebar
entry.