Argparse library improvements, cross-compilation support, and several compiler fixes.

[cs01]

Summary

Argparse library improvements, cross-compilation support, and several compiler fixes.

Argparse

• Add subcommand support (parser.addSubcommand()) and migrate chad-native.ts CLI to use it
• Add --key=value equals syntax for options
• Add -- double-dash support to stop flag parsing
• Simplify setFlagValue/setOptionValue to use direct arr[i] = value instead of rebuilding arrays

Cross-compilation

• Add --target flag with short names (linux-x64, macos-arm64, etc.) and full LLVM triple support
• Thread target triple through opt, llc, and clang invocations
• Use target OS (not host) for platform-specific IR symbols and linker flags
• Add --target-cpu flag and default CI to x86-64 to fix SIGILL on runners
• Add cross-compile CI workflow that builds on Linux→macOS and macOS→Linux, then links and runs on the target

Compiler fixes

• Enum values: getEnumMemberValue() was returning the loop index instead of the declared value — enum { OK = 200 } produced 0 instead of 200
• Discriminated unions: Generalize type guard narrowing to work with any discriminant field, not just type. Enables patterns like if (shape.kind === "circle").
• Union common fields: getUnionCommonFields was returning TS types instead of LLVM types, causing type mismatches

Other

• Site and build fixes
• Installer updates

Test plan

• All 295 unit tests pass (including 4 new: enum values, 3 discriminated union variants)
• Self-hosting verification passes (Stage 0 + Stage 1)
• New argparse fixtures: subcommand, equals syntax, double-dash
• New enum fixture: explicit values (200, 404, 500), auto-increment, mixed
• New union fixtures: basic discriminant, function params, custom kind field