feat: add authz schema policy renderer

[rodmgwgu]

Problem

A compiled schema carries bare identifiers; the enforcer stores namespaced Casbin p rows. Something has to translate between the two, and it has to be the single place that applies the namespacing — two places would drift, and a later comparison against the stored policy would silently stop matching (ADR 0018 §5).

Approach

• openedx_authz/engine/renderer.py — PolicyRow, RenderedPolicy, policy_row(), PolicyRenderer
• openedx_authz/tests/schema/test_renderer.py

render() emits one p row per (role, permission, supported scope), applies the internal form (role^, act^, <scope>^*) at this boundary, and is deterministic so the output can be diffed against a stored policy. It emits definition rows only — never g assignments or legacy g2 action inheritance — so data owned by other services is out of reach by construction.

The render phase is pure: no Casbin, no Django, no database. The apply phase, which does touch both, is the next PR in the stack.

Manual testing instructions

pytest openedx_authz/tests/schema/test_renderer.py

Rollback plan

Revert this PR. Nothing consumes the renderer yet, so the revert is inert.

Retro compatibility

No authorization behavior changes. Rendering produces rows in memory and writes nothing.

AI Usage

Kiro was used to assist on feature planning and implementation. Implementation was done step by step with human guidance and validation, based on the ADRs.

---

Stack (6/8) — #446 split into reviewable pieces. Bases chain bottom-up; merge in order.

1. feat: add authz schema discovery #474 — schema discovery
2. feat: add authz schema loading #475 — schema loading
3. feat: add authz schema compilation #476 — schema compilation
4. feat: add authz schema validation #477 — schema validation
5. feat: add authz schema definition models #478 — definition models + migration
6. This PR — policy renderer (base: rod/authz-schema-models)
7. rod/authz-schema-applier — schema applier
8. feat: add authz schema pipeline and load_authz_schema command #446 — pipeline + load_authz_schema command, version bump and changelog

In #446 render and apply shared one 686-line module. They are split here so the pure, easily-reviewed render step is not bundled with the transactional apply step; 7/8 adds the rest of the same file.

Merge checklist:

• Version bumped — deferred to 8/8 so the stack does not conflict on every restack
• Changelog record added — deferred to 8/8
• Documentation updated (not only docstrings) — covered by the already-merged ADRs
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets — depends on feat: add authz schema definition models #478; no migration

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[mariajgrimaldi]

Just a few comments for you to review!

[mariajgrimaldi]

Don't have this already defined in data.py?

[mariajgrimaldi]

[non-blocking] Could we reuse the existing policy parsing here? PolicyIndex already defines these field positions, and get_permission_from_policy() parses and validates the same list shape. It may be worth extracting shared parsing so the mapping and validation stay in one place.

[mariajgrimaldi]

This looks more than a constructor that should be part of PolicyRow?