[MAINTENANCE] Unify RealSim→FIXS branding in source code, logs, and comments

[yunlishao-vibe]

Task Summary

The codebase uses "RealSim" and "FIXS" inconsistently throughout source code, log messages, comments, and configuration keys. Standardize on "FIXS" as the product name everywhere.

Why is this needed?

The project was formerly called "RealSim" and was rebranded to "FIXS", but the rename was never propagated through the source code. This causes confusion for new contributors and makes documentation searches inconsistent.

Examples of current inconsistency:

• SocketHelper.cpp: logs print "RealSim start server task", "RealSim Initialized"
• VirEnvHelper.cpp: "RealSim shutdown", "Warning: RealSim shutdown failed"
• ConfigHelper.cpp: EnableRealSim config key, "RealSimPOI_" prefix
• ConfigHelper.h: struct field EnableRealSim
• TrafficHelper.cpp: vehicle type set to "RealSim"
• PerformanceTimer.h: log file named "RealSimPerf.log"
• RealSimVersion.h.in: macro prefix REALSIM_VERSION_*
• RealSimSocket.cpp: S-function named RealSimSocket
• MATLAB files: RealSimSocket, RealSimDepack, RealSimPack, RealSimMsgClass, RealSimInitSimulink, RealSimInterpSpeed, RealSimTemplate, RealSimTemplate_dSPACE
• requirements.txt still references old version numbers

Scope of Work

1. Source code (C++)

• Replace "RealSim" log strings with "FIXS" in SocketHelper.cpp, VirEnvHelper.cpp, ConfigHelper.cpp, TrafficHelper.cpp
• Rename EnableRealSim → EnableFIXS in struct and config parsing (backward-compat: accept both keys during transition)
• Rename RealSimPOI_ prefix → FIXS_POI_
• Rename RealSimPerf.log → FIXS_Perf.log
• Update REALSIM_VERSION_* macros in RealSimVersion.h.in → FIXS_VERSION_*

2. Config YAML files

• Update all config.yaml test files: rename EnableRealSim → EnableFIXS
• Add backward-compat parsing so old configs still work

3. Python helpers

• Rename RealSimSocket references in SocketHelper.py, MsgHelper.py
• Update VehDataMsgDefs.py if any RealSim references exist

4. Documentation and comments

• Sweep all inline comments using "RealSim" and update to "FIXS" (keep historical context where relevant)

5. Auto-test

• Add tests/Python/test_naming_consistency.py that greps source files for legacy "RealSim" strings and fails if found in key locations (excluding MATLAB API files which must keep backward compat and files under `archive/`)

Files Involved

• CommonLib/SocketHelper.cpp, VirEnvHelper.cpp, ConfigHelper.cpp, ConfigHelper.h, TrafficHelper.cpp
• CommonLib/PerformanceTimer.h, CommonLib/RealSimVersion.h.in
• All tests/**/config.yaml files

Acceptance Criteria

• All runtime log messages use "FIXS" not "RealSim"
• Config key EnableRealSim replaced with EnableFIXS (with backward compat)
• FIXS_VERSION_* macros replace REALSIM_VERSION_*
• tests/Python/test_naming_consistency.py passes and enforces the standard
• No functional regression (all existing tests still pass)

Branch and PR Target

• All PRs for this issue should target the dev branch (not main)
• First ensure main is merged into dev before starting work