fix(embedded): Windows protoc fails with extended-length path from canonicalize() #39

Problem

Building `coordinode-embedded` on Windows fails because `canonicalize()` in `coordinode-raft/build.rs` returns a Windows extended-length path (UNC prefix `\\?\`) that protoc does not support:

```
Error: protoc failed: Invalid file name pattern or missing input file
"\\?\D:\a\...\proto/coordinode/v1/replication/raft.proto"
```

`std::path::Path::canonicalize()` on Windows produces `\\?\D:\...\path` which is a verbatim UNC prefix. protoc does not handle this prefix and rejects the path.

Suggested fix

In `coordinode-rs/crates/coordinode-raft/build.rs`, strip the `\\?\` prefix on Windows before passing the path to protoc:

```rust
#[cfg(target_os = "windows")]
fn strip_unc_prefix(p: std::path::PathBuf) -> std::path::PathBuf {
let s = p.to_string_lossy();
if let Some(stripped) = s.strip_prefix(r"\\?\") {
std::path::PathBuf::from(stripped)
} else {
p
}
}
```

Or use the `dunce` crate (already common in Rust build scripts) which simplifies `\\?\` paths portably.

Files involved

• `coordinode-rs/crates/coordinode-raft/build.rs`

Context

Observed in CI: build job `build-embedded / windows-latest` after merging PR #37.