Land feature 57's SDK work on its release branch

Providers, the state bus, templates and the environment collection that the
feature's modules build on (feature 57, config and provisioning).

go/stdio.go

@@ -66,6 +66,8 @@ type channel struct {
 	subscribed int
 	// watches are this bundle's state watches by the id each was asked with (ADR 0201).
 	watches map[string]func(StateChange) error
+	// provisioned are this bundle's watches of its values, by the id each was asked with (provision.go).
+	provisioned map[string]func(ProvisionChange) error
 	// takers and provers are this bundle's seat work and proof handlers, by worker and subject (ADR 0259).
 	takers  map[string]func(Work) error
 	provers map[string]func(subject string, body json.RawMessage) (any, error)
@@ -169,7 +171,8 @@ func ServeOn(name string, tools []Tool, in io.Reader, out io.Writer) error {
 		byName[t.Name] = t
 	}
 	c := &channel{enc: json.NewEncoder(out), waiting: map[string]chan message{}, watches: map[string]func(StateChange) error{},
-		takers: map[string]func(Work) error{}, provers: map[string]func(string, json.RawMessage) (any, error){}}
+		provisioned: map[string]func(ProvisionChange) error{},
+		takers:      map[string]func(Work) error{}, provers: map[string]func(string, json.RawMessage) (any, error){}}
 	currentMu.Lock()
 	current = c
 	currentMu.Unlock()
@@ -297,6 +300,28 @@ func ServeOn(name string, tools []Tool, in io.Reader, out io.Writer) error {
 				}
 				c.say(map[string]any{"id": id, "result": map[string]any{}})
 			}(id, *p.Change)
+		case "mesh/provision":
+			// One change to a value of this module's that this bundle watches (provision.go), named by the
+			// watch's id, answered once its handler has taken it. Beside the loop, like a tool call.
+			var p struct {
+				Watch  string           `json:"watch"`
+				Change *ProvisionChange `json:"change"`
+			}
+			_ = json.Unmarshal(m.Params, &p)
+			c.mu.Lock()
+			handle := c.provisioned[p.Watch]
+			c.mu.Unlock()
+			if handle == nil || p.Change == nil {
+				c.say(map[string]any{"id": id, "error": map[string]any{"code": -32602, "message": "mesh/provision names no watch of this bundle"}})
+				continue
+			}
+			go func(id any, change ProvisionChange) {
+				if err := handle(change); err != nil {
+					c.say(map[string]any{"id": id, "error": map[string]any{"code": -32000, "message": err.Error()}})
+					return
+				}
+				c.say(map[string]any{"id": id, "result": map[string]any{}})
+			}(id, *p.Change)
 		case "mesh/work":
 			// One piece of a seat's work this bundle takes (novox/hq ADR 0259): taken by an answer that is
 			// not an error, offered again after one. Beside the loop, like a tool call.