Pin Core to its merged main history

[tconley1428]

Python's Core submodule still points to the pre-merge head of #1625 (9af4a6d5), which is outside Core main after that PR was squash-merged. This moves the pin to main commit 26ed8876, restoring a normal ancestry relationship for future Core updates.

The changelog tooling is unchanged between these pins. The target also contains the merged MSRV resolver fix. Regenerate the four affected Python bridge protobuf bindings with poe gen-protos-docker to include Core's child-workflow versioning override and new child-start failure causes. The bridge lockfile needs no update. The new Core-update and release-note flow follows in stacked #1936.

Validation:

• Confirmed the target is an ancestor of Core main and the changelog-release-notes sources match the old pin.
• Bridge Cargo metadata resolves with the existing lockfile.
• 16 release-helper tests pass.
• poe bridge-lint passes.
• poe build-develop and poe gen-protos-docker complete successfully, without changing dependency files.
• Generated bindings exactly match the patch produced by CI's check-protos job; the new field roundtrip and failure-cause values are verified.