Comparer: derivable removals and changelog layout

[VisLab]

No description provided.

[Copilot]

🟡 Changes recommended

It introduces a non-robust assert in a unittest and has a couple of output/formatting stability and convention issues that should be fixed before merging.

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Pull request overview

Updates the schema comparison/reporting logic to better align with HED 8.5.0 unit derivation behavior and to emit changelog output in the same markdown layout used by hed-schemas prerelease notes, with supporting tests and minor downstream call-site/doc updates.

Changes:

• Treat removed units as a Patch change when the newer schema still accepts them via derivation (e.g., uV as u + V).
• Reformat SchemaComparer.pretty_print_change_dict() markdown output to match hed-schemas prerelease/PRERELEASE_CHANGES.md layout and extend compare_differences() to support non-markdown output.
• Add/adjust tests for derived-unit defaultUnits validation and the new comparer behaviors/layout.

File summaries

File	Description
hed/schema/schema_comparer.py	Adds derivable-unit removal classification and updates changelog formatting + API.
hed/scripts/schema_script_util.py	Forces plain-text schema diff output for reload error reporting.
hed/schema/schema_validation/compliance_summary.py	Updates compliance summary bullet list to reflect current rules.
tests/schema/test_schema_compare.py	Adds regression tests for derivable unit removals and markdown layout expectations.
tests/schema/test_schema_attribute_validators.py	Adds test coverage for derived-form defaultUnits validation.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 3
• Review effort level: Lite

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