Fix S-param mode not reloading on project deserialize

[striderZA]

Summary

Fixes #44. deserialize() in the five S-param-capable engines (Amplifier, IdealFilter, Equalizer, Attenuator, Combiner) restored sparam_mode/sparam_filepath from the project JSON but never reloaded the Touchstone file. update() gates the S-param path on sparam_mode && sparam_data.loaded(), so a reloaded project silently fell back to ideal/manual mode while the UI still showed S-param mode as enabled.

Root cause

Confirmed by reading each engine's deserialize(): the loader (m_sparam_data.load(...) / m_sparam.load(...)) was only ever invoked from the setSParamFilepath()/setSParamFile() setters used by live UI interaction — never from deserialize(), which is also the path used for project load, component cloning, and library-part instantiation.

EqualizerEngine::serialize() additionally omitted sparam_fwd_idx, which its S-param branch (equalizer_engine.cpp:51) depends on.

Fix

• amplifier_engine.cpp, ideal_filter_engine.cpp, equalizer_engine.cpp: deserialize() now reloads m_sparam_data from the deserialized filepath and gates m_sparam_mode on the actual loaded() result (so a stale/missing file now correctly reports mode off instead of on-but-broken). sparam_fwd_idx is preserved from JSON, not recomputed.
• attenuator_engine.cpp, combiner_engine.cpp: same pattern via m_sparam.load(...).
• equalizer_engine.cpp serialize(): added missing sparam_fwd_idx field.

Test

Added tests/test_project_file.cpp case 13 ("Round-trip: S-param mode survives save/load (issue #44)"): builds all 5 component types in S-param mode using the existing component_data/amplifiers/adm-3844psm/*.s2p fixture, saves, reloads in a fresh RfSimulatorApp, and asserts sparamMode()/sparamLoaded()/sParamMode() are all true post-reload.

Verification

• Full tests suite: 216 test cases, 65530 assertions — pass, no regressions
• [project_file] filter: 12 cases, 90 assertions — pass (includes new test)
• [sparam] filter: 12 cases, 4057 assertions — pass
• test_attenuator.exe / test_combiner.exe — pass
• Formatted with clang-format-18 (matches CI)