Provide the libxml2 link dependency for native Windows ARM64 LLVM

[LunaStev]

Summary

The native Windows ARM64 compiler build cannot link against the official LLVM 21.1.8 ARM64 package because llvm-sys requests xml2s.lib, but that library is not available in the configured LLVM/MSVC library paths.

This currently blocks both the native Windows ARM64 build job and the Windows ARM64 cases job introduced by #474.

Reproduction

Run the native Windows ARM64 jobs from #474 with:

cargo build --locked --release --target aarch64-pc-windows-msvc \
  --no-default-features --features llvm-target-aarch64 --jobs 2

Observed linker failure:

LINK : fatal error LNK1181: cannot open input file 'xml2s.lib'

Affected CI jobs:

• Build Windows ARM64 natively
• Cases Windows GNU arm64 (the job currently runs on a native ARM64 Windows host despite its legacy display name)

Expected behavior

The native Windows ARM64 compiler links successfully using an explicitly provisioned, checksum-pinned libxml2 dependency or an LLVM distribution/link mode that does not leave xml2s.lib unresolved.

Acceptance criteria

• Document why LLVM's ARM64 MSVC package requests xml2s.lib in this configuration.
• Provision or avoid the dependency without using an unpinned binary download.
• Keep the compiler and dependency architecture native ARM64.
• Make both Windows ARM64 build and cases jobs reach their Wave validation steps.
• Add a CI assertion that catches a mismatched x64/ARM64 dependency early.

Evidence

• PR: feat: expand native 64-bit architecture support #474
• Build job: https://github.com/wavefnd/Wave/actions/runs/33944526111/job/101248104258
• Cases job: https://github.com/wavefnd/Wave/actions/runs/33944526096/job/101248122649

[LunaStev]

Native verification from PR #486 head 6fb6ed7: the pinned libxml2 build verifies 44 ARM64 objects; the native release compiler now links successfully, Clippy passes and all three Rust unit tests pass. The original missing xml2s.lib error is no longer the stopping point.

Native build job, native cases job. Subsequent failures are tracked separately: #492 for HOME/std-directory discovery and #493 for 113 access violations in cases. Those must be resolved before claiming the full native lane green; this issue remains open until the linked PR is merged.