Fix Windows Git dependency paths

[LunaStev]

Summary

• normalize Windows verbatim disk and UNC paths before passing managed dependency repositories to Git
• generate platform-correct, percent-encoded local file: URLs for Git integration fixtures
• run the complete Git integration suite on Windows x64 instead of unit tests only
• move macOS Apple Silicon from Intel cross-build coverage to native macos-15 tests and build
• update official checkout/setup actions to their Node 24-compatible v7 releases
• document v0.0.1 candidate and experimental platform validation tiers

Why

Rust canonicalization on Windows returns extended-length paths such as \\?\C:\.... Git for Windows rejects those paths as clone destinations, while the existing fixture helper also produced malformed file://C:\... URLs. As a result, Windows CI could only run unit tests and did not verify fetch, lockfile reproduction, offline mode, or targeted update behavior.

Behavior

• Vex keeps canonical paths internally but converts verbatim disk and UNC prefixes only at the Git subprocess boundary.
• local Git fixtures use file:///C:/... or file://server/share/... URLs and percent-encode reserved characters.
• fixture directories contain spaces and #, exercising URL decoding through real clone/fetch/update operations.
• Windows x64 and both macOS architectures now run the full native Rust test suite.
• v0.0.1 uses the Windows MSVC target; Windows GNU is out of scope. RISC-V remains experimental.

Validation

• python3 x.py check
• cargo check --locked --all-targets --target x86_64-pc-windows-msvc
• python3 -m unittest discover -s tests/xpy -v
• workflow YAML parse
• git diff --check
• PR CI: all eight jobs passed, including native Windows and macOS arm64