test(journeys): a walrus binds to its nearest scope, not to the whole subtree

[Yambr]

Follow-up to #430, from a review pass on the merged guard.

The defect

The walrus collector swept the entire comprehension with ast.walk, so a := written inside a nested lambda was attributed to the enclosing function and reddened that function's clean list argv:

def f(x):
    cmd = ["docker", "ps"]
    fns = [lambda: (cmd := f"rm {x}") for _ in range(1)]
    subprocess.run(cmd)          # flagged, and clean

PEP 572 binds a walrus in the scope that contains it — inside a lambda that is the lambda, not the function. At runtime cmd is still ["docker", "ps"].

The fix

The collector recurses through direct children and stops at every nested scope, the same pruning the nested-def fix applied. A walrus written in the comprehension itself still binds outward and is still caught, in both list and generator forms.

Status

Latent, not live. The harness contains no walrus expression today, so the tree was 0-hit either way. It is worth closing because it is the same reds-on-the-safe-form failure the earlier scoping work drove out, and a guard that reds on the safe form forces the per-site waivers off instead of keeping them honest.

Measured

• walrus inside a lambda: clean; walrus in the comprehension: caught; genexp form: caught; plain if (cmd := ...): caught
• harness 32 passed / 1 skipped, semgrep tree 0
• mutation: restoring the subtree walk reds 14 cases

One note on method: my first mutation probe of this failed to parse, so its green measured a build error rather than the guard. Redone with the indentation preserved before the result was read.

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