Allow walk to take an integer depth

[anyingiit]

Description

Project(walk=...) (and Project.resolve(walk=...)) now also accepts a non-negative integer,
giving the maximum number of directory levels to descend below the root, as proposed in #69:

• walk=True – walk the whole tree (unchanged)
• walk=False / walk=0 – never descend (unchanged for False)
• walk=None (default) – descend one level only if the root matched no spec (unchanged)
• walk=N – descend at most N levels; each child is scanned with N - 1

Because bool is a subclass of int, booleans are handled first. There is no extra validation of
the value (per review). The two TODO comments about an integer walk are removed,
scan_glob's type hint/docstring is updated (it passes walk straight through), and the intro docs
mention walk=2. A directory without any spec still counts as one level (its matches show up as
"dir/sub" children as before).

I did not change the CLI (--walk is still a flag) or switch to fsspec's depth-first walk for early
pruning (the "possible addition" in the issue) — happy to follow up on either if wanted.

Motivation / disclosure: I had some spare AI-assistant quota (Claude Code) and am using it to try to
help projects with open good-first-issues. The change was prepared with Claude Code and verified as listed
below. If it doesn't fit, isn't up to your bar, or you'd simply rather not take it — please feel free to
close it, no hard feelings at all 🙂

Related issue

Closes #69

Checklist

• Tests pass locally (pip install -e .[test], pytest -q: 589 passed, 16 skipped, with cargo on PATH for the Rust round-trip tests; with the source reverted to main, pytest tests/test_basic.py -k walk gives 3 failed, 7 passed (the integer-depth cases); pre-commit run on the changed files passes)
• CHANGELOG.md is updated (if applicable) — n/a (the repo has no changelog)
• Documentation is updated (if applicable) — docstrings of Project, Project.resolve, scan_glob, and docs/source/intro.rst

[anyingiit]

Thanks! Both done in 809fee8: the type guard (and its test) is gone, and the child's walk value is now computed inline where the child project is created.