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/atlas/build_test.go

@@ -0,0 +1,48 @@
+package atlas_test
+
+import (
+	"reflect"
+	"testing"
+
+	"git.d-ma.be/mathias/cad-atlas/internal/atlas"
+)
+
+func TestBuild_OverlaysCIStageNodesFromWorkflow(t *testing.T) {
+	atlasJSON := []byte(`{
+	  "substrate": [{"n":"koala","k":"gpu"}],
+	  "stages": [
+	    {"no":"STAGE 06","title":"PR → CI","generate":"ci-jobs","nodes":[]},
+	    {"no":"STAGE 01","title":"TELOS","nodes":[{"t":"Intention substrate"}]}
+	  ]}`)
+	workflow := []byte("jobs:\n  check:\n    a: 1\n  build:\n    b: 2\n  deploy:\n    c: 3\n")
+
+	a, err := atlas.Build(atlasJSON, workflow)
+	if err != nil {
+		t.Fatalf("Build: %v", err)
+	}
+
+	// CI stage nodes generated from the workflow's jobs, in order.
+	var titles []string
+	for _, n := range a.Stages[0].Nodes {
+		titles = append(titles, n.Title)
+	}
+	if want := []string{"check", "build", "deploy"}; !reflect.DeepEqual(titles, want) {
+		t.Fatalf("CI stage nodes = %v, want %v", titles, want)
+	}
+
+	// Non-generated stage is untouched.
+	if a.Stages[1].Nodes[0].Title != "Intention substrate" {
+		t.Fatalf("authored stage was altered: %+v", a.Stages[1])
+	}
+
+	// Substrate preserved.
+	if len(a.Substrate) != 1 || a.Substrate[0].Name != "koala" {
+		t.Fatalf("substrate not preserved: %+v", a.Substrate)
+	}
+}
+
+func TestBuild_ErrorsOnBadAtlasJSON(t *testing.T) {
+	if _, err := atlas.Build([]byte("{not json"), []byte("jobs:\n  x:\n    a: 1\n")); err == nil {
+		t.Fatal("expected error on bad atlas JSON, got nil")
+	}
+}