Phase 3: add reld-owned Rust bindings and route Windows through vendored llvm-ld

[zackees]

Parent: #92
Blocked by the standalone zackees/llvm-ld COFF library issue.

Vendor the pinned C/C++ library below _vendor/ through a git-backed import and keep all Rust crate/API ownership inside reld.

Requirements:

• No crates.io publication or dependency.
• Add a narrow safe Rust wrapper over the C ABI.
• Integrate MSVC and GNU/MinGW routing while retaining the pinned direct lld-link oracle/escape path.
• Require deterministic, byte-identical EXE and PDB comparison versus pinned stock LLD, with identical args/path/cwd/environment.
• Run native Windows acceptance with exact output for both personas; cross-compilation alone is insufficient.
• Add allocator performance/profile measurements before linker algorithm changes.
• Update Windows guidance and relevant architecture docs.
• One validated PR; close this issue on merge.

[zackees]

Gate (owner decision, via #154): this waits for #153 — the #123 no-gaps gate that unblocks soldr — before it starts, as #154's sequencing says. When it starts, the C++ build dependency it needs still requires the owner's explicit approval under AGENTS.md's dependency policy, requested on this issue before any work begins.

Upstream is ready: every prerequisite from #95 is delivered in zackees/llvm-ld (#95 closed with the evidence), including the narrow C ABI and its contract tests.