docs(versioning): state the rules for what changes between releases

.changeset/versioning-three-rules.md

@@ -0,0 +1,5 @@
+---
+"adcontextprotocol": patch
+---
+
+Docs: state the rules for what changes between releases at the top of the versioning page. A minor only adds, a replaced field is emitted alongside its replacement until the next major, and removals and breaking changes happen in a major. Removes text from that page that told SDKs what to do or described translating between versions, and corrects one sentence in the `create_media_buy` reference that said new keys could join a closed schema in a minor. Wire behavior is unchanged.

docs/media-buy/task-reference/create_media_buy.mdx

@@ -381,7 +381,7 @@ When `confirmed_at` is `null`, sellers MUST omit `packages[].committed_metrics`.
   - **`attribution_methodology`** — when `metric_id` is an outcome metric (`conversions`, `conversion_value`, `roas`, `cost_per_acquisition`, `incremental_sales_lift`, `brand_lift`, `foot_traffic`, `conversion_lift`, `brand_search_lift`, `units_sold`, `new_to_brand_rate`, `new_to_brand_units`, `leads`) and the seller commits to a specific attribution methodology, the entry SHOULD carry `qualifier.attribution_methodology` (`deterministic_purchase` for retail-media closed-loop; `probabilistic`, `panel_based`, or `modeled` for other paths). Two outcome rows under different methodologies are not interchangeable; symmetric on `missing_metrics`.
   - **`attribution_window`** — when `metric_id` is an outcome metric and the seller commits to a specific lookback window, the entry SHOULD carry `qualifier.attribution_window` as a structured duration (`{ interval: 14, unit: "days" }`). Two outcome rows over different windows are reported as separate rows so buyers don't accidentally aggregate across periods.
 
-  Without the qualifier, the contract is ambiguous and reconciliation falls back to whatever the delivery report happens to carry. The qualifier vocabulary is closed (`additionalProperties: false`); new keys ship explicitly in subsequent minors.
+  Without the qualifier, the contract is ambiguous and reconciliation falls back to whatever the delivery report happens to carry. The qualifier vocabulary is closed (`additionalProperties: false`), so new keys wait for the next major.
 
   Vendor-scope entries MAY carry the same qualifier keys when the same vendor metric is committed under more than one methodology or window — e.g., `attribution_window` on a vendor outcome metric distinguishes a 14-day vendor attribution commitment from a 30-day one for the same `(vendor, metric_id)`. `missing_metrics` mirrors the qualifier for vendor entries exactly as it does for standard entries.
 - **Reconciliation:** `missing_metrics` on [`get_media_buy_delivery`](/docs/media-buy/task-reference/get_media_buy_delivery) filters `committed_metrics` to entries where `committed_at < reporting_period.end`, then flags any that aren't populated in the report. A metric committed mid-flight is only audited from its commitment timestamp forward. Qualifiers are matched verbatim — a committed `{viewable_rate, mrc}` is not satisfied by a delivered `viewable_rate` carrying `viewability.standard: groupm`.