docs and fix: Clarify that yaml priority only applies to role_extensions

[rodmgwgu]

Closes #525

Problem

priority is meant to resolve conflicts between role_extensions only. The compiler also applied it to base definitions: when two files declared the same category, permission, or role differently, the higher-priority file silently won. This was never in an ADR and contradicted validation, which already rejects such conflicts regardless of priority.

Approach

• Added ADR 0031 stating priority resolves role extensions only, never base definitions.
• Compiler: differing base definitions now raise SchemaCompileError regardless of priority (identical ones still merge). Extension resolution is unchanged.
• Clarified the priority docs in ADR 0017, ADR 0023, the schema reference, and the JSON Schema.

Manual testing instructions

In a tutor dev env with this repo mounted:

1. tutor dev run cms ./manage.py cms load_authz_schema --dry-run — applies cleanly (shipped schema has no conflicts).
2. Declare the same role ID in two files with different content and priorities, then run load_authz_schema — expect a CommandError reporting the conflict; DB untouched.
3. Replace the second definition with a role_extensions entry — expect success, confirming priority still resolves extensions.

Automated: TestBaseDefinitionConflicts covers the new behavior; full schema suite (278 tests) passes.

Rollback plan

Revert this PR. Contained to the compiler, its tests, and docs. No migration.

Retro compatibility

No behavior change for supported deployments: validation already rejected these conflicts before compile. Only a direct compiler call with conflicting base definitions now raises instead of picking a winner.

AI Usage

Kiro assisted with investigation, ADR authoring, and implementation, with human guidance on the decision and validation against the ADRs.

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Merge checklist:

• Changelog record added — Fixed entry under Unreleased
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets — no migration

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