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/model.go

@@ -47,11 +47,13 @@ type Stage struct {
 
 // Atlas is the full data model the frontend renders.
 type Atlas struct {
-	Version   string   `json:"version,omitempty"`
-	Substrate []Host   `json:"substrate"`
-	NS        string   `json:"ns,omitempty"`
-	Timeline  []RunDot `json:"timeline,omitempty"`
-	Stages    []Stage  `json:"stages"`
+	Version     string   `json:"version,omitempty"`
+	Substrate   []Host   `json:"substrate"`
+	NS          string   `json:"ns,omitempty"`
+	Timeline    []RunDot `json:"timeline,omitempty"`
+	CIDurationS float64  `json:"ci_duration_s,omitempty"`
+	CDDurationS float64  `json:"cd_duration_s,omitempty"`
+	Stages      []Stage  `json:"stages"`
 }
 
 // Build unmarshals the authored atlas JSON and overlays generated facts from