fix: crash/robustness — malformed project/library JSON, S-param path containment, touchstone OOM caps

[striderZA]

Summary

Crash/robustness fixes from a comprehensive codebase review (findings C1, B7, S1, S2).

• Malformed project JSON crashes the app (C1). ProjectSerializer::load() caught only JSON parse errors; wrong-shape-but-valid JSON ({}, "components": 5, wrong-typed params, non-object window_state) threw uncaught nlohmann exceptions and std::terminated. Now: 64 MiB size cap before parse, root must be an object, the whole load body is guarded (json::exception → log + graceful failure), and each component deserialize is isolated so one bad component is skipped while the rest load. Same crash class fixed at the library boundary (component_library loadFile catches type_error; scan tolerates filesystem_error).
• Coax preset OOB (B7). deserialize clamped preset_index/length_m/connectors_loss_dB — previously an out-of-range index into kCoaxCablePresets from a corrupted project was UB.
• S-param path containment (S1). S-param paths in project files now resolve against the project file's directory and must stay inside it (weakly_canonical + prefix, mirroring the extension manifest parser). Absolute/escaping paths are neutralized with a warning. Library data_files entries get the same containment. Save re-relativizes in-project absolute paths so projects stay portable.
• Touchstone OOM caps (S2). Parser rejects > 256 MiB files before reading and enforces the 10M-point cap during the read loop instead of after buffering (~960 MB OOM window removed).

Verification

• tests.exe: 218 test cases / 65534 assertions, all pass (216 baseline + 2 new wrong-shape JSON cases).
• New standalone test_path_containment exe: 5 cases / 29 assertions.
• Full build green (MinGW-w64, Debug, ninja).

Notes for merging

• Contains a behavior change: absolute S-param paths outside the project dir are neutralized at load — keep .s2p data inside the project directory. Relative paths now resolve against the project dir (portable) instead of the process CWD.
• Expected conflict with refactor/engine-unification in tests/test_project_file.cpp (the Fix S-param mode not reloading on project deserialize #56 test's attenuator/combiner lines: this branch stages the fixture with the old API names, that branch renames the API). Merge this branch first; resolve by keeping setSParamFilepath(local_s2p) / sparamMode().