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/web/handler.go

@@ -55,6 +55,7 @@ func NewHandler() http.Handler {
 					a.Stages[i].Nodes = nodes
 				}
 			}
+			a.CIDurationS, a.CDDurationS = atlas.StageSeconds(*ld.run)
 		}
 		if ld.issues != nil {
 			for i := range a.Stages {