Evaluate stack policy against the effective base and emit stack events #52

Outcome

Policy for every stacked PR evaluates against the stack's trunk, never against an intermediate feature branch, and stack membership is visible in payloads and events.

Context

docs/stacked-prs.md sections 5.7–5.8, 11.1–11.2, and 16 define the split. A lower PR that edits CODEOWNERS or workflow definitions must not weaken requirements for the layers above it before it lands. The forge has no webhooks yet; event delivery uses the transactional outbox pattern already present (repository_provisioning_outbox, graph_mutation_outbox) plus PubSub broadcasts.

Acceptance criteria

• Stack evaluation carries both bases: the direct base for diffs and display, the effective trunk base for policy resolution.
• Policy configuration resolves from the effective base tree, not the parent branch tree; after a layer lands on trunk, later evaluations see the new policy legitimately.
• PR payloads include stack context: number, position, size, health, and the effective base ref and OID.
• The event catalog covers stack created, appended, restructured, dissolved, rebase started/conflicted/completed, and merge lifecycle events, each carrying stack id and number, operation id, version, actor, old and new ordering, and affected head OIDs.
• Events persist through an outbox with consumer deduplication by event id.

Verification

Tests proving a CODEOWNERS edit in position 1 does not change requirements at position 2 pre-merge, payload shape assertions, and outbox delivery tests. Run mix precommit.

Dependencies

Stack schema (#47). Branch protection and rulesets do not exist yet on the forge; this issue delivers the plumbing and consumes them when they land.