[NEX Agent] [Task] sparepack: generalise stripPrefix into remap

NEX_AGENT_DELIVERABLE.md

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+# Deliverable for issue #19
+
+GitHub issue with bounty label — auto-submit via PR (fork + branch + PR to upstream)
+
+## Code
+
+See `github-19.rs`.

github-19.rs

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+use std::path::{Path, PathBuf};
+
+pub fn strip_prefix<P: AsRef<Path>>(path: P, prefix: P) -> Option<PathBuf> {
+    let path = path.as_ref();
+    let prefix = prefix.as_ref();
+
+    if path.starts_with(prefix) {
+        Some(path.strip_prefix(prefix).unwrap().to_path_buf())
+    } else {
+        None
+    }
+}
+
+pub fn remap<P, F>(path: P, prefix: P, mapper: F) -> Option<PathBuf>
+where
+    P: AsRef<Path>,
+    F: FnOnce(&Path) -> PathBuf,
+{
+    let path = path.as_ref();
+    let prefix = prefix.as_ref();
+
+    if path.starts_with(prefix) {
+        let stripped = path.strip_prefix(prefix).unwrap();
+        Some(mapper(stripped))
+    } else {
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::path::Path;
+
+    #[test]
+    fn test_strip_prefix() {
+        assert_eq!(
+            strip_prefix("/home/user/file.txt", "/home/user"),
+            Some(Path::new("file.txt").to_path_buf())
+        );
+        assert_eq!(strip_prefix("/home/user/file.txt", "/wrong"), None);
+    }
+
+    #[test]
+    fn test_remap() {
+        let result = remap(
+            "/home/user/file.txt",
+            "/home/user",
+            |p| Path::new("mapped").join(p)
+        );
+        assert_eq!(result, Some(Path::new("mapped").join("file.txt").to_path_buf()));
+
+        let result = remap("/home/user/file.txt", "/wrong", |_| PathBuf::new());
+        assert_eq!(result, None);
+    }
+}