add plan for describe override-axis rollup

[HumanBean17]

Adds execution-ready implementation plan for propose/DESCRIBE-OVERRIDE-ROLLUP-PROPOSE.md.

Contents

• New file: plans/PLAN-DESCRIBE-OVERRIDE-ROLLUP.md — single-PR breakdown, principles, tests (verbatim names from propose), coordination with landed PR-89 _edge_summary_for_node branch, regression note for test_describe_method_symbol_no_composed_keys.

Behaviour

• Docs/planning only — no code, ontology, or re-index changes.

Validation

• Not applicable (markdown plan only).

[HumanBean17]

Review (planning doc only)

Overall: This is a solid execution plan: it tracks the propose’s surface semantics, pins the merge point in _edge_summary_for_node, calls out the PR-89 interaction (disjoint kind branches), and the regression note for test_describe_method_symbol_no_composed_keys is exactly the right guardrail once override-axis keys exist.

Strengths

• Clear file-by-file breakdown and a concrete DoD checklist.
• Explicit handling of UC8 (static), UC11 (constructors as no-op), edge-row vs distinct-method counting for brownfield projections, and UC13 single-hop scope.
• Fixture/oracle strategy matches existing patterns in test_mcp_v2_compose.py (e.g. test_describe_class_with_brownfield_clients_emits_composed_key).

Small fixes worth a follow-up commit on this branch

• Typo: “type-rolloup” → “type-rollup” in the regression-guard subsection.
• Cross-doc consistency: propose/DESCRIBE-OVERRIDE-ROLLUP-PROPOSE.md §6 still says new tests live in tests/test_mcp_v2.py; this plan correctly targets tests/test_mcp_v2_compose.py — consider a one-line note in the propose when implementation lands, or align the propose in a tiny doc PR so implementers do not split tests across files by mistake.

Implementation nudge (for PR-1)

• Current _edge_summary_for_node is type-branch-only; adding elif kind == "symbol" and … after the existing if preserves the “never both rollups” invariant for normal rows. Worth double-checking the dispatch-up Cypher also respects the static suppression for OVERRIDES (whole-helper gate vs per-branch), as the plan already flags.

Meta: GitHub shows mergeable state as unstable — if that persists, a rebase on latest master after other describe work lands may be needed before merge.

LGTM as a planning artifact; no blocking issues.