feat(19.7-02): pier MCP module tracer with in-process dispatch and sessions

- modules/pier: Host contract, mcp.* config, catalog compiled from
  pact.HasMCP (prefixing, route binding, guard resolution, schema
  reflection, annotations), per-request identity through the published
  verifier and resolvers, built-in backend guard provider, in-process
  dispatch with the bouncer marker, per-identity SDK server with the
  per-request re-filter, stateful session router with rehydration over a
  memory store, audit lines
- modules/surf/pier_on.go: the summercms_mcp hook adapting the router to
  pier.Host; the untagged hello binary carries no SDK
- go.mod: go-sdk v1.8.0 and jsonschema-go v0.4.3 (decision note committed
  before this change)
- TestPierTracerSmoke: SDK client lists and calls acme_echo end to end
  through surf's compiled guard chain; README and root table row

modules/surf/pier_on.go

@@ -0,0 +1,68 @@
+//go:build summercms_mcp
+
+package surf
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+	"git.golem15.com/golem15/summercms/modules/party"
+	"git.golem15.com/golem15/summercms/modules/pier"
+)
+
+// Binaries built with the summercms_mcp tag (MCPBuildTag) link the MCP
+// module: this file sets the mount hook BuildRouter calls when mcp.enabled
+// is true, with an adapter that gives pier the route table, the compiled
+// handler lookup and the two mount kinds without pier importing surf.
+func init() {
+	mountPier = func(r *Router, app *backpack.App, plugins []party.Plugin) error {
+		return pier.Mount(pierHost{r: r}, app, plugins)
+	}
+}
+
+// pierHost adapts a Router to pier.Host.
+type pierHost struct {
+	r *Router
+}
+
+func (h pierHost) Routes() []pier.RouteInfo {
+	routes := h.r.Routes()
+	out := make([]pier.RouteInfo, len(routes))
+	for i, ri := range routes {
+		out[i] = pier.RouteInfo{Method: ri.Method, Pattern: ri.Pattern, PluginID: ri.PluginID, Middleware: ri.Middleware}
+	}
+	return out
+}
+
+func (h pierHost) Handler(method, pattern string) (http.Handler, bool) {
+	return h.r.Handler(method, pattern)
+}
+
+func (h pierHost) MountStream(method, path string, handler http.Handler, middleware ...string) error {
+	h.r.BindPlugin(pier.PluginID)
+	return h.r.MountStream(method, path, handler, middleware...)
+}
+
+// MountJSON registers a plain GET route (no middleware) under the pier
+// pseudo-plugin; only GET is supported.
+func (h pierHost) MountJSON(method, path string, handler http.Handler) error {
+	if strings.ToUpper(strings.TrimSpace(method)) != http.MethodGet {
+		return fmt.Errorf("surf: pier may mount JSON routes with GET only, got %q", method)
+	}
+	if owner, ok := h.Owner(http.MethodGet, path); ok {
+		return fmt.Errorf("surf: route GET %s (plugin %q) collides with a route the MCP module mounts", path, owner)
+	}
+	h.r.BindPlugin(pier.PluginID)
+	h.r.GroupRaw("", nil, func(g pact.Router) {
+		g.Get(path, handler.ServeHTTP)
+	})
+	return nil
+}
+
+func (h pierHost) Owner(method, path string) (string, bool) {
+	owner, ok := h.r.seen[strings.ToUpper(strings.TrimSpace(method))+" "+path]
+	return owner, ok
+}