Harden CLI parsing and enable unsafe-buffer diagnostics

[netliomax25-code]

This PR improves the security hardening posture of the project by removing raw argv range pointer arithmetic from CLI parsing paths and enabling Clang's -Wunsafe-buffer-usage diagnostics where supported.

The patch replaces direct pointer-range construction patterns such as:

args.assign(argv + 1, argv + argc);

with explicit bounds-checked iteration using standard containers.

In addition, this PR introduces regression coverage for ArgumentParser::ParseCLI(int, const char* const*) and enables compiler-assisted detection of unsafe buffer usage patterns.

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Security Rationale

This change improves the auditability and safety of CLI argument parsing by:

• Eliminating raw pointer range arithmetic in parser code paths
• Improving compatibility with Clang Safe Buffers analysis
• Enabling -Wunsafe-buffer-usage diagnostics to help prevent future regressions
• Handling malformed or partially invalid argv entries more defensively
• Making bounds reasoning explicit and easier to review

The hardening approach aligns with modern secure-by-design buffer handling practices and supports ongoing migration away from unsafe buffer manipulation idioms.

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Changes

args.hxx

• Replaced raw argv + 1, argv + argc vector assignment with explicit iteration
• Added defensive null checks before appending CLI arguments
• Reserved vector capacity before insertion to avoid repeated reallocations

examples/gitlike.cxx

• Replaced raw argv pointer-range construction with explicit indexed iteration
• Added defensive handling for null argument entries

CMakeLists.txt

• Enabled -Wunsafe-buffer-usage when supported by Clang
• Updated comments to reflect removal of previously incompatible raw pointer idioms

Tests

Added a regression test covering:

• ArgumentParser::ParseCLI(int, const char* const*)

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[Taywee]

See #173 and CONTRIBUTING.md:

• Do not make defensive changes for conditions that will never happen. Examples:
  • Don't check for null pointers for things that can not be null, like argv[i]
    where i is in the range [0, argc). If the C standard guarantees a shape for
    argv and arc, we don't need to additionally check those ourselves.

If you want to be able to enable -Wunsafe-buffer-usage, I'm not against a change that disables that for the section, but I am against defensively protecting against something that will never happen.