🔖 Prepare release of v1.4.2

[burgholzer]

🤖 AI text below 🤖

Description

Consolidate the merged v1.4.2 and v1.4.3 release preparations into the single unpublished v1.4.2 release.

PR #383 prepared v1.4.2 but the corresponding tag and release were never published. PR #384 then prepared v1.4.3 under the mistaken assumption that v1.4.2 had already been released. This corrective PR:

• folds the MLIR template change from 🔖 Prepare release of v1.4.3 #384 into the v1.4.2 changelog and upgrade-guide sections;
• removes the premature v1.4.3 sections and links; and
• restores pyproject.toml and uv.lock to version 1.4.2.

The existing v1.4.2 draft release remains unpublished and targets main, so it can be published after this PR is merged.

Validation

• uv lock
• uv run --group test python -m pytest (9 passed)
• uv run prek run --all-files
• Independent exact-candidate verification (9 passed; no v1.4.3 references; no scope expansion)

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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