A build says what it does on the bus, as it happens (novox/hq ADR 0157)

The build-machine seat emits `started` and `log.<build id>` beside `built`. Every line the builder
speaks — each step, each command with its duration, and on failure the command's own output — goes
to stderr as before and onto the bus under the build's id, one subject per build, kept a week in
EVENTS with every other event. `builds --log <id>` reads it back from the stream with a consumer
that is gone when the reading is done, on the command line and as the controller's seat verb;
`builds` lists each build's id and `build` says the id it asked with.

Lines are core publishes with a sequence number, so a build is not slowed by an ack per line and a
gap is visible; `started` and `built` are awaited into the stream. The seat protocol widens
additively at the controller's next start; the holder's grant follows on the broker node's next
composition.

internal/link/builds.go

@@ -27,6 +27,40 @@ const TheBuildMachine = "mesh-build-machine"
 func BuildWork() string    { return "mesh.seat." + TheBuildMachine + ".accept.build" }
 func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.built" }
 
+// BuildStarted is where a build machine says it has taken a build, and BuildLog is where it says
+// what it is doing, one line per message, under the build's own id (novox/hq ADR 0157).
+//
+// **The whole build is on the bus as it happens.** The outcome alone told a person that a build
+// failed and its first line why; everything between — which command, how long, where it hung —
+// lived in one container's stderr on one machine. Every line is now an event of the role, retained
+// with the rest of the mesh's events, so a reader follows a build live by subscribing its subject,
+// or reads it back afterwards from the stream, and a viewer is a subscriber and nothing more.
+func BuildStarted() string      { return "mesh.seat." + TheBuildMachine + ".event.started" }
+func BuildLog(id string) string { return "mesh.seat." + TheBuildMachine + ".event.log." + id }
+
+// BuildStart is what a build machine says the moment it takes a build.
+type BuildStart struct {
+	ID         string `json:"id"`
+	Repository string `json:"repository"`
+	Path       string `json:"path,omitempty"`
+	Ref        string `json:"ref,omitempty"`
+	On         string `json:"on"`
+	At         string `json:"at"`
+}
+
+// BuildLine is one thing a build said while building.
+type BuildLine struct {
+	ID string `json:"id"`
+	// Seq counts the lines of one build from 1, so a reader that joined late or read two copies
+	// can order them and see a gap.
+	Seq int    `json:"seq"`
+	At  string `json:"at"`
+	// Step is which part of the build spoke — clone, context, image, run, failed — and Message is
+	// what it said, as the builder's own log prints it.
+	Step    string `json:"step"`
+	Message string `json:"message"`
+}
+
 // KeyRoleBuilt is the build outcome under the role's name, on the bus the mesh runs on today.
 //
 // The same event as KeyModuleBuilt and published beside it, because a catalogue installed before this
@@ -57,6 +91,13 @@ type BuildMachine interface {
 
 // Build is one request a machine has been handed.
 type Build interface {
+	// Began says the build has been taken and is under way, before anything runs.
+	Began(ctx context.Context) error
+
+	// Say publishes one line of what the build is doing. Never fails the build: a line the bus
+	// did not take is a line lost, and the outcome still comes.
+	Say(step, message string)
+
 	// Request is what to build.
 	Request() BuildRequest