Add DragonFly BSD amd64 support with native runtime verification

The BSD work currently tracks FreeBSD (#413, #386), OpenBSD and NetBSD (#471), but DragonFly BSD is absent from the target registry and case manifest.

Add the amd64 DragonFly target using authoritative DragonFly source/headers and LLVM support. Implement its own executable linker/startup/sysroot handling and std providers; do not treat DragonFly as FreeBSD or Linux. Verify syscall/error conventions, time and socket layouts and filesystem behavior. Provide a VM-based build and runtime job and package verification once runnable.

Completion requires meaningful shared cases and DragonFly-specific runtime/error-path cases to execute, C ABI interoperability, and clear diagnostics for missing sysroots. Preserve unrelated architecture and Windows GNU support. Additional architectures require actual OS support rather than placeholder entries.

Related: #413, #386, #471.

Start by locating the target registry and case manifest, then compare the existing FreeBSD, OpenBSD, and NetBSD entries and their shared cases. Done means amd64 DragonFly support has its own linker, startup, sysroot, and std handling, VM build/runtime and package verification, C ABI coverage, DragonFly-specific runtime/error cases, and clear missing-sysroot diagnostics.

Written by the indexing model from the issue text.