docs(runway): state the merge target atomicity guarantee in the contract

api/runway/messagequeue/README.md

@@ -25,6 +25,8 @@ One message serves a queue pair because a merge-conflict check is a dry run of a
 
 `MergeResult.outcome` is an `Outcome` enum (`OUTCOME_UNSPECIFIED`/`SUCCEEDED`/`FAILED`): `SUCCEEDED` means mergeable (check) or merged (commit), `FAILED` a conflict or a failed apply; `reason` carries the explanation when `FAILED`. Per-step detail is in `steps` (request order): each `StepResult.outputs` is the list of `StepOutput`s the step produced on the merge target, **in application order** (the order they were created). A committing merge populates `outputs`; a dry-run check, an already-present change, or a failed step leaves them empty. `StepOutput.id` is the VCS-neutral revision identifier (git SHA, Mercurial hash, Subversion revision, Perforce changelist, …), with room to grow (author, timestamp, …).
 
+A committing merge is all-or-nothing against the merge target: Runway updates the target at most once per request, and a `SUCCEEDED` result means every step in `steps` is reachable from the target, while a `FAILED` result means the target is unchanged.
+
 ## Evolution
 
 Contract changes are additive-only: add new fields; never remove, rename, repurpose, or retype an existing field, and never reuse a field number. protojson ignores unknown fields on read and omits zero-valued fields on write, so a new optional field is backward-compatible in both directions.

doc/rfc/runway/workflow.md

@@ -79,6 +79,8 @@ Together these guarantee the client's correlation id always resolves: the primar
 
 Runway has no persistent state — no request store, no job store, no database. Idempotency is achieved through the VCS contract: merge detects already-pushed changes (revisions reachable from HEAD) and treats them as already-landed. Merge-conflict check is read-only and naturally idempotent.
 
+A committing merge is also atomic against the merge target: it updates the target at most once per request, and afterwards either every step of the request is reachable from the target or the target is unchanged. A retried redelivery therefore either replays cleanly against the same unchanged target or finds its work already landed.
+
 ## Ownership by service
 
 ### Runway

runway/extension/merger/merger.go

@@ -55,6 +55,9 @@ type Merger interface {
 	CheckMergeability(ctx context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error)
 	// Merge applies the ordered steps, commits the result to the remote, and
 	// reports per-step Outputs (the VCS-neutral revision identifiers produced).
+	// Merge is all-or-nothing against the target: it updates the target at
+	// most once per request, and afterwards either every step is reachable
+	// from the target or the target is unchanged.
 	Merge(ctx context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error)
 }