[M4.2] Harden LLVM target lowering and linker integration

[xeondesk]

Goal

Make native code generation correct and diagnosable for the supported target contract.

Scope

• Consume resolved target triples and data layouts without reparsing CLI strings.
• Emit valid LLVM for all MIR instructions and supported control flow.
• Add linker argument construction, object/executable naming, and tool failure capture.
• Inspect generated symbols and sections in tests; reject unsupported cross-target builds clearly.

Acceptance criteria

• Host builds are runnable and Clang-parseable.
• LLVM verification or equivalent validation runs before linking.
• Link failures include the exact target and tool invocation context without leaking secrets.

Dependency

#2, #3, #22