Bump stdlib version to 0.5.7

[dioderobot]

Automated PR to update the pinned stdlib version to 0.5.7.

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Note

Low Risk
Single-constant version bump plus changelog update; risk is limited to any upstream stdlib behavior changes impacting builds at runtime.

Overview
Updates the pinned stdlib version used as the implicit minimum dependency by changing pcb-zen-core::STDLIB_VERSION from 0.5.6 to 0.5.7.

Adds an Unreleased changelog entry noting the stdlib bump.

^{Written by Cursor Bugbot for commit 8e62d0c. This will update automatically on new commits. Configure here.}