Publish only a commit on its module's trunk; post a pull request's change plan (hq ADR 0238)

One commit, one plan: a commit off the trunk — a pull request's head, a branch built by
hand, a rebuild or replay of one — is for checking. The build seat reads from its clone
which branches hold the commit, and the controller records and never registers a build
whose commit is not on the branch the module follows (the repository's default for a
new one), so nothing off the trunk can be sent.

A pull request's check now carries its change plan, computed by the planner: what a
merge would build in which order, what each machine would receive, and what is not an
ordinary send — the bus step, a module waiting for a person, a provider's consumers.

internal/link/build.go

@@ -114,6 +114,8 @@ type CheckRequest struct {
 	// Dependents are the modules a merge would build after Modules because they stand on them: the
 	// planner's dependency walk, said on the pull request.
 	Dependents []string `json:"dependents,omitempty"`
+	// Plan is the change plan of the commit checked, computed by the controller and echoed with the outcome.
+	Plan *ChangePlan `json:"plan,omitempty"`
 	// Manifests are the touched modules' manifests in the change's tree, by path from its root: what the
 	// gate puts through `module check`.
 	Manifests []string `json:"manifests,omitempty"`
@@ -203,6 +205,15 @@ type BuildResult struct {
 	// manifest the mesh keeps says nothing about it (novox/hq 04-ISSUES/131).
 	Read []ReadRepository `json:"read,omitempty"`
 
+	// Trunk is the repository's default branch at the build, and OnTrunk whether the commit built is on it
+	// (novox/hq ADR 0238): **only a commit on the trunk is published** — the controller refuses to register
+	// a build of one off it. Empty Trunk is a build seat that could not say, or predates the rule.
+	Trunk   string `json:"trunk,omitempty"`
+	OnTrunk bool   `json:"on-trunk,omitempty"`
+	// Branches are every branch the commit is on: the trunk of a module that follows a branch other than
+	// the repository's default is the branch it follows.
+	Branches []string `json:"branches,omitempty"`
+
 	// SourceFingerprint is what the build was made from, hashed (novox/hq issue 280): the module's
 	// tree at the commit, the trees of the contexts it read, its bases and toolchains by digest. Two
 	// builds with one fingerprint are one build, however their digests differ — an image is not

internal/link/events.go

@@ -258,6 +258,41 @@ type Checked struct {
 	// RepoCheck is nil when the repository is not the mesh's and touches nothing of it: nothing is said.
 	Gate      *CheckLayer `json:"gate,omitempty"`
 	RepoCheck *CheckLayer `json:"repo-check,omitempty"`
+	// Plan is the change plan of the commit checked (novox/hq ADR 0238): what a merge of it would build
+	// and send, posted with the verdict.
+	Plan *ChangePlan `json:"plan,omitempty"`
+}
+
+// ChangePlan is what a change does to the mesh, computed from its diffset — a repository, the branch it
+// merges into and the commit at hand — by the planner (novox/hq ADR 0238): **one commit, one plan**, the
+// object a pull request's check posts, the release follows and a person reads.
+type ChangePlan struct {
+	Repository string `json:"repository"`
+	Base       string `json:"base"`
+	Head       string `json:"head"`
+	// Moved are the modules a merge moves itself, Dependents those built after them because they stand
+	// on them, New the directories it adds a module in, and Unread the changed files no build reads.
+	Moved      []string `json:"moved,omitempty"`
+	Dependents []string `json:"dependents,omitempty"`
+	New        []string `json:"new,omitempty"`
+	Unread     []string `json:"unread,omitempty"`
+	// Tiers are the build plan's order: each tier built after the one before.
+	Tiers [][]string `json:"tiers,omitempty"`
+	// Machines are the deploy plan: what each machine is sent, in the build plan's order, and what waits
+	// there for a person.
+	Machines []MachinePlan `json:"machines,omitempty"`
+	// Steps are what is not an ordinary send: a planned bus step, a provider whose consumers are sent again,
+	// a module that waits for a person, a module that keeps data.
+	Steps []string `json:"steps,omitempty"`
+	// Summary is the plan in one line, for a commit status.
+	Summary string `json:"summary"`
+}
+
+// MachinePlan is one machine's part of a change plan.
+type MachinePlan struct {
+	Machine  string   `json:"machine"`
+	Receives []string `json:"receives,omitempty"`
+	Waits    []string `json:"waits,omitempty"`
 }
 
 // CheckLayer is one layer of a merge check, judged.