fix(cli): resolve a relative paths.data against the project root

Env::data_path() handed paths.data to the data layer verbatim, so a
relative value resolved against the invoking process's cwd — aura run
from a project subdirectory silently read a different archive location
than from the root, while runs/ stayed anchored. Relative values now
join onto the project root, matching runs_root() and the [nodes]
pointer resolution; absolute values and the no-project default pass
through unchanged. RED-first unit test pins the root-anchored
resolution.

closes #254

crates/aura-cli/tests/common/mod.rs

@@ -69,8 +69,9 @@ fn mint_tempdir() -> PathBuf {
 /// Writes the `Aura.toml` + `demo_signal.json` common to every fresh project:
 /// an absolute `[nodes]` pointer at the shared, once-built fixture crate, and
 /// — when `data_dir` is `Some` — a `[paths] data` override pointing at it
-/// (relative, resolved against the invocation cwd, which every test that uses
-/// this always sets to `root` itself via `.current_dir`).
+/// (relative, resolved against the project root — `root` itself — since
+/// `Env::data_path` joins a relative `paths.data` onto the project root,
+/// same as `runs_root()` and the `[nodes]` pointers, #254).
 fn write_project_files(root: &Path, data_dir: Option<&str>) {
     let fixture = built_project();
     let data_line = data_dir.map(|d| format!("data = \"{d}\"\n")).unwrap_or_default();