v1.4.0: Harden path traversal check, fix handler double-write and getPathType panic
- Security: inTrustedRoot now requires path under root with separator
- Fix: return nil after 404 to avoid second response write
- Fix: getPathType returns pathTypeFile on os.Stat error (avoid nil deref)
- Fix: handle ReadDir error in makeFeed; use log in verifyPath
- Typos and test cleanup (filepath.Join, Test_absoluteCanonicalPath)