[ty] Avoid order-dependent inference for guarded attributes

[charliermarsh]

Summary

We infer implicit instance attributes from assignments such as:

class C:
    def __init__(self):
        if not hasattr(self, "x"):
            self.x = self.__str__

A negative hasattr guard normally narrows self to Self & ~Protocol[x]. When the same guard initializes x, simplifying that type requires resolving x again, creating a Salsa cycle whose recovered result depends on file-checking order rather than constraint-set ordering.

Avoid the cycle by leaving self unnarrowed only when existing use-def reachability proves that the same standalone guard initializes the same implicit instance attribute in the corresponding branch. Preserve normal narrowing for class-backed attributes, previously initialized attributes, compound conditions, and assignments in the opposite branch.

Stacked on #25645.

Closes astral-sh/ty#4076.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 96.96%. The percentage of expected errors that received a diagnostic held steady at 92.96%. The number of fully passing files held steady at 106/133.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
unused-type-ignore-comment	0	2	0
unresolved-attribute	0	1	0
Total	0	3	0

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Raw diff:

websockets (https://github.com/aaugustin/websockets)
- src/websockets/trio/client.py:543:50 error[unresolved-attribute] Object of type `Self@__aenter__` has no attribute `nursery`

werkzeug (https://github.com/pallets/werkzeug)
- src/werkzeug/debug/__init__.py:315:55 warning[unused-type-ignore-comment] Unused blanket `type: ignore` directive
- src/werkzeug/debug/__init__.py:331:55 warning[unused-type-ignore-comment] Unused blanket `type: ignore` directive

Full report with detailed diff (timing results)