Add a file context that sits in front of another one and delegates every
primitive call to it.
The wrapper adopts the identity of the wrapped context: uri, root, chroot
flag, and the chroot path composition are the wrapped context's, so a path
composed by the wrapper composes identically in the wrapped context. Every
primitive is forwarded to the wrapped context's primitives, which receive
the already-composed path; open() and close() are forwarded to the wrapped
context's public methods, so the open count is kept in one place.
The point is to give derived contexts a stable base: a caching context, or
any other filter, overrides the primitives it cares about and inherits the
rest, instead of re-deriving the chroot bookkeeping of every method.
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v1.1.0
Signed-off-by: Jan Lindemann <jan@janware.com>