fix(fetch): create destination parents for nested downloads

[DivyamTalwar]

Summary

Fixes #9.

The normal CLI dispatch calls fetch.run without creating dest, unlike the standalone module entry point. open(part) fails for a new destination or a nested remote filename. Create the output parent before HTTP/writing, retaining ordinary errors when a file blocks that directory.

Testing

Base: 252e5193902d466726da9af75047dfffff2ae662. Debian 12 Linux aarch64 in a nonroot disposable container, Python 3.11. The full existing smoke exits 0 with eight pre-existing optional research/hardware skips, explicitly not counted as passes. No Windows run or GPU/real-model benchmark is claimed.

The same final test files fail against unchanged production; the corrected branch gives:

Focused: 6 passed in 0.19s
Full applicable suite: 8 SKIPPED (a skip is not a pass):
all green

python tests/smoke.py
ruff check quantprobe
ruff format --check quantprobe
# Bandit medium-severity checks on changed package modules
git diff --check

All applicable commands above exited 0 locally. The snapshot is a complete upstream checkout plus this branch's exact changed-file bytes; no test is a copied production-function reimplementation. External I/O is mocked where stated. Dependencies and lockfiles are unchanged.

Security And Data Access

No credential, production-data, authentication, read-only guardrail or privileged workflow changes are included. Tests use synthetic inputs and disposable paths. No new benchmark, fitted law, or hardware capability is claimed.

Notes

The change repairs fetch and its actual CLI dispatch, not auto's separate basename bookkeeping after nested downloads. Path interpretation is unchanged; no path-containment guarantee is introduced. Failed attempts may leave empty directories.

AI-assisted implementation, isolated same-provider source review, and controller regression checks are disclosed. They are not maintainer approval, cross-vendor certification, or hosted CI. One focused, signed-off commit; no generated logs, personal config, model weights or worktree state is included.

Draft pending upstream CI and maintainer review. Companion changes touching the same module/test hook may require rebasing as they land; no combined branch is being submitted.