fix ReadDir at local fs, relax path req for compatibility (#54)

(fix) Inconsistent file path behaviour (#53)

Key Changes:
Updated shared validation functions (filename.go):

IsValidFile(), IsValidPath(), and IsValidDir() now accept paths with or without leading /
Both /file.txt and file.txt are treated as equivalent paths relative to the mount point
Relative paths with .. are still rejected by fs.ValidPath()
Both filesystems now behave identically:

S3 filesystem (stream): Already had path normalization in s3Key() that strips leading /
Local filesystem (lfs): Uses trim() function to normalize paths before filesystem operations
Both accept the same path formats and treat them consistently
Updated documentation:

Both Create() and Open() methods in both filesystems now document that paths can be with or without leading /
Clear that both forms are treated as paths relative to the mount point

Before:
Paths required leading / → fsys.Create("/file.txt", nil) ✅
Paths without / failed → fsys.Create("file.txt", nil) ❌

After:
Both forms work equally → fsys.Create("/file.txt", nil) ✅
→ fsys.Create("file.txt", nil) ✅
Both are treated as the same file at the mount root
No support for relative traversal with ../ (correctly rejected)

(fix) return error from spool foreach (#51)

(fea) CurlFS as an abstraction for pre-signed URLs (#49)

(fea) CurlFS as an abstraction for pre-signed URLs (#49)

adapt spool api for io.Writer, eliminate buffering

Unify spool api (#47)

log spool error (#46)

support spool partition (#45)

mount s3 at root and path prefix (#44)