FIX: sys_read/sys_write must pass u64::MAX for non-positioned I/O

The default kreadoff/kwriteoff implementations in scheme/mod.rs require
offset=u64::MAX for stream (non-positioned) I/O. Passing offset=0 causes
ESPIPE (Illegal seek), which breaks ALL writes to non-positioned FDs
including Debug/stdout. This was the root cause of the procmgr child
appearing to 'panic' — its log output silently failed with ESPIPE.

src/syscall/fs.rs

@@ -832,7 +832,7 @@ pub fn sys_read(fd: FileHandle, buf: UserSliceWo, token: &mut CleanLockToken) ->
             let offset = if desc.internal_flags.contains(InternalFlags::POSITIONED) {
                 desc.offset
             } else {
-                0
+                u64::MAX
             };
             Ok((
                 scheme.kreadoff(desc.number, buf, offset, desc.flags, desc.flags, token)?,
@@ -861,7 +861,7 @@ pub fn sys_write(fd: FileHandle, buf: UserSliceRo, token: &mut CleanLockToken) -
             let offset = if desc.internal_flags.contains(InternalFlags::POSITIONED) {
                 desc.offset
             } else {
-                0
+                u64::MAX
             };
             Ok((
                 scheme.kwriteoff(desc.number, buf, offset, desc.flags, desc.flags, token)?,