feat(ui): render the rail timeline as a component behind its builder

The observed-history transport is RailTimeline in src/ui/rail/, rendered
by createRailTimeline() with the same update and destroy API. Drag preview,
the coalesced 150 ms commit, release, cancel and blur behave as before; the
slider keeps the ticks it is dragged over, an update from inside a preview
cannot move the handle back, and readouts follow the handle within the same
task. utcClock moves to its own module and is still exported here.

scripts/package-boundaries.json

@@ -573,6 +573,8 @@
       "src/ui/rail/RailCardBlocks.vue",
       "src/ui/rail/RailCard.vue",
       "src/ui/rail/RailCards.vue",
+      "src/ui/rail/utcClock.js",
+      "src/ui/rail/RailTimeline.vue",
       "src/ui/rail/rangeValue.js"
     ],
     "external": ["vue"]
@@ -938,6 +940,8 @@
       "src/ui/rail/RailCardBlocks.vue",
       "src/ui/rail/RailCard.vue",
       "src/ui/rail/RailCards.vue",
+      "src/ui/rail/utcClock.js",
+      "src/ui/rail/RailTimeline.vue",
       "src/ui/rail/rangeValue.js"
     ],
     "external": ["cesium", "vue", "pinia"]
@@ -2898,6 +2902,8 @@
       "src/ui/rail/RailCard.vue",
       "src/ui/rail/RailCards.vue",
       "src/ui/rail/RailCardBlocks.vue",
+      "src/ui/rail/utcClock.js",
+      "src/ui/rail/RailTimeline.vue",
       "src/ui/rail/rangeValue.js"
     ],
     "external": [