feat(atlas): Phase B — data-driven /api/atlas.json, CI stage generated from cd.yml

The atlas no longer hand-maintains its content. Authored data lives in one place
(internal/atlas/atlas.json); the Go layer overlays sourced facts and serves the
result at /api/atlas.json; the frontend fetches + renders (inline arrays kept only
as an offline fallback). First generated source: the CI/CD stage's nodes are parsed
from the repo's own .gitea/workflows/cd.yml — so the viz shows the pipeline that
actually runs (guard/check/build/deploy), dropping the aspirational var-go/oath-gate
node that isn't wired yet. That's the point: it can't drift from the real pipeline.

New internal/atlas package (JobsFromWorkflow, Build) built test-first. Adds
gopkg.in/yaml.v3 (justified: parsing the workflow YAML; stdlib has no YAML).

Verified: go build/vet/lint(0)/test green; /api/atlas.json → 9 stages, CI = real
jobs; frontend renders from the fetch (screenshot).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/handler.go

@@ -1,20 +1,22 @@
 package web
 
 import (
-	"context"
 	_ "embed"
+	"encoding/json"
 	"net/http"
+
+	cadatlas "git.d-ma.be/mathias/cad-atlas"
+	"git.d-ma.be/mathias/cad-atlas/internal/atlas"
 )
 
-// atlasHTML is the Phase-A static hero visualization. Phase C replaces this
-// self-contained file with a Templ view hydrated from live CAD trace data
-// (assessor-loop ledger, session_log, Gitea run API, Flux events).
+// atlasHTML is the Phase-A/B static shell. It fetches /api/atlas.json at load
+// and renders from that data (no inline arrays), so the content is sourced.
 //
 //go:embed static/cad-atlas.html
 var atlasHTML []byte
 
-// NewHandler serves the CAD Atlas. Root ("/") returns the static atlas;
-// /api/hello is a leftover template probe kept until Phase C wires real endpoints.
+// NewHandler serves the CAD Atlas: the shell at "/", and the sourced data at
+// "/api/atlas.json" (authored data + CI stage generated from the real cd.yml).
 func NewHandler() http.Handler {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
@@ -25,8 +27,14 @@ func NewHandler() http.Handler {
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
 		_, _ = w.Write(atlasHTML)
 	})
-	mux.HandleFunc("/api/hello", func(w http.ResponseWriter, r *http.Request) {
-		_ = Hello("world").Render(context.Background(), w)
+	mux.HandleFunc("/api/atlas.json", func(w http.ResponseWriter, r *http.Request) {
+		a, err := atlas.Default(cadatlas.CDWorkflow)
+		if err != nil {
+			http.Error(w, "atlas build failed", http.StatusInternalServerError)
+			return
+		}
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		_ = json.NewEncoder(w).Encode(a)
 	})
 	return mux
 }