Add loader write controls and once mode

[Poyraxx]

Summary

• add a write_mode = "once" option so existing dumps are preserved without appending or truncating
• let gluasteal.lua return a second boolean to control whether the current file should be written to disk
• add gluasteal.getenv and loader-local os.getenv(...) compatibility for standalone Lua loader scripts
• document the new loader behavior and add a small parsing test for the new write mode

Why

Issue #49 is still open in the current code because only truncate and append exist, and the loader cannot suppress writes per file. A later report in issue #73 also showed that os.getenv is missing inside the loader environment.

Validation

• configured with Visual Studio Build Tools CMake + Ninja
• built successfully on Windows x64
• ctest --test-dir build --output-on-failure