feat(atlas): weight replay pacing on CI/CD stages by real job durations (#5)

TDD: Job.Seconds() + StageSeconds() derive real CI/CD dwell time from the
latest run's per-job created_at/updated_at (last job in pipeline order =
deploy/stage 07, everything before it = CI/stage 06).

Frontend: weightedSpineDist() redistributes the pixel-time-budget the
06/07 segments already had, splitting it by real CI:CD duration ratio
instead of raw pixel width. Falls back to the exact prior constant-speed
sweep when no run data is available (weights default to segment pixel
length) or when a job is skipped (0 duration) — no behavior change for
stages 00-05/08, which still have no live timing source (same gap as #5's
ledger item).

Verified: real duration values flow through /api/atlas.json (ci_duration_s:
18 observed against a real run), full page screenshot confirms no visual
regression.

internal/atlas/runs_test.go

@@ -2,6 +2,7 @@ package atlas_test
 
 import (
 	"testing"
+	"time"
 
 	"git.d-ma.be/mathias/cad-atlas/internal/atlas"
 )
@@ -104,3 +105,52 @@ func TestLatestRunJobs_ErrorsWhenEmpty(t *testing.T) {
 		t.Fatal("expected error on empty, got nil")
 	}
 }
+
+func TestLatestRunJobs_ParsesPerJobTimestampsIntoSeconds(t *testing.T) {
+	tasks := []byte(`{"workflow_runs":[
+	  {"run_number":28,"name":"Deploy via GitOps","status":"success","created_at":"2026-07-20T21:05:44Z","updated_at":"2026-07-20T21:05:50Z"},
+	  {"run_number":28,"name":"Lint / Test / Vet","status":"success","created_at":"2026-07-20T21:05:39Z","updated_at":"2026-07-20T21:05:44Z"}
+	]}`)
+
+	s, err := atlas.LatestRunJobs(tasks)
+	if err != nil {
+		t.Fatalf("LatestRunJobs: %v", err)
+	}
+	// pipeline order: Lint first, Deploy last
+	if got := s.Jobs[0].Seconds(); got != 5 {
+		t.Fatalf("Lint job seconds = %v, want 5", got)
+	}
+	if got := s.Jobs[1].Seconds(); got != 6 {
+		t.Fatalf("Deploy job seconds = %v, want 6", got)
+	}
+}
+
+func TestStageSeconds_LastJobIsDeploySumOfRestIsCI(t *testing.T) {
+	s := atlas.RunSummary{Jobs: []atlas.Job{
+		{Name: "Lint", Started: mustParse("2026-07-20T21:00:00Z"), Finished: mustParse("2026-07-20T21:00:10Z")},   // 10s
+		{Name: "Build", Started: mustParse("2026-07-20T21:00:10Z"), Finished: mustParse("2026-07-20T21:00:25Z")},  // 15s
+		{Name: "Deploy", Started: mustParse("2026-07-20T21:00:25Z"), Finished: mustParse("2026-07-20T21:00:33Z")}, // 8s
+	}}
+	ci, cd := atlas.StageSeconds(s)
+	if ci != 25 {
+		t.Fatalf("ci = %v, want 25", ci)
+	}
+	if cd != 8 {
+		t.Fatalf("cd = %v, want 8", cd)
+	}
+}
+
+func TestStageSeconds_NoJobsReturnsZero(t *testing.T) {
+	ci, cd := atlas.StageSeconds(atlas.RunSummary{})
+	if ci != 0 || cd != 0 {
+		t.Fatalf("ci=%v cd=%v, want 0,0", ci, cd)
+	}
+}
+
+func mustParse(s string) time.Time {
+	t, err := time.Parse(time.RFC3339, s)
+	if err != nil {
+		panic(err)
+	}
+	return t
+}