Add timeout_ms to fd, walk, ls, and async path functions

[ncoop57]

Summary

Path-finding functions (fd, walk, ls, fda) now accept timeout_ms, returning partial results with stop_reason=\"timeout\" — matching the existing behavior on rg/rga/nbrg/nbrga.

Changes

• walk.rs: Refactored find/find_cancelable into a unified find_iter built on the generic StreamIter/spawn_walk engine, eliminating the bespoke channel/worker code. find is now a thin collect() over the iterator.
• python.rs: walk_py, find_py, and find_async_py route through collect_stream_py / stream_async with deadline support, returning (Vec<String>, bool) tuples so Python can set stop_reason.
• __init__.py: Added timeout_ms param to walk, fd, and fda; results wrapped via _mk_results to carry the timeout flag.
• Docs: Updated DEV.md and README.md to document timeout_ms on all path functions.
• Tests: Added timeout assertions for fd, walk, and fda.
• fd_iter/fda_iter: Lazy path walks over the same find_iter engine, yielding FileEntry rows. Adds the FindIter pyclass and _core.find_iter/find_iter_async in python.rs.