AdCP 3.3: semantic downgrade rules for 3.3-only fields toward 3.2 peers #1667

Spec

3.3 adds fields and values inside objects that 3.2 closes, so they must be sent only to peers that negotiated 3.3 (migration guide §Send only to 3.3 peers, tag v3.3.0-beta.0). DR-0024 also requires responses to conform to the served release. compat/versions.py projects known newer fields generically through both schemas once #1662 lands, but some rules need semantics, not just field removal:

• by_spot[]: scheduled / preempted rows are omitted, not stripped of spot_status, because 3.2 requires aired_at.
• Delivery rows without impressions: below 3.3 the row is omitted, never sent as impressions: 0.
• media_buy.features.catalog_ingestion: coarsened or dropped on 3.2 capability projections. Capabilities can't be gated, so check the buyer side parses tolerantly.
• reporting_webhook.authentication / artifact_webhook.authentication: absent on a 3.3 request, required below 3.3. The downgrade fails structured (3.2 to 3.1 conversion emits payloads 3.1 refuses: empty notification event_types, and geo_regions as {} #1621).
• daypart_targets[].start_time / end_time: no 3.2 form. Refuse rather than round.
• age_determination_bases: the interim ext.adcp.age_determination_bases for 3.2 peers; top-level wins on read.
• operator_fee, REFERENCE_DEFINITION_CHANGED, new enum values: the downgrade maps or drops them.

Work (after #1662)

For each rule: a conversion test at 3.3 → 3.2, plus a fix where the generic projection does the wrong thing. Pins, expected_confirmation_at and pixel trackers are tracked separately in #1641, #1640 and #1643.

Inventory: docs/adcp-3.3-sdk-inventory.md §3, §4. Batch C.