fix(flights): keep ground-snap sampling after the clock steps back (#1399)

Recover aircraft ground-height sampling after backward wall-clock changes while preserving sampling budgets.

Contribution by @quanticsoul4772.

docs/CURRENT-STATE.md

@@ -5309,6 +5309,11 @@ pins and share links already supply complete boxes.
 
 - **The TRACKED contact's 2D↔3D handoff is DEFAULT behaviour (2026-08-19), driven by camera distance alone.** It does NOT consult the DISPLAY-rail `3D` toggle, which continues to own the FLEET (the un-instanced draw-call budget stays the operator's decision). Policy lives in `src/data/trackedModelRegime.js` and is shared by both layers: enter below `TRACKED_MODEL_ENTER_ALT_M` = 150,000 m and hand back to the billboard only above `TRACKED_MODEL_EXIT_ALT_M` = 172,500 m. **The swap distance was set by playtesting on 2026-08-20:** a first pass at 1,000,000 m switched too early; 2D reads correctly at ~600 km and the handoff belongs at ~150 km. **Consequence, recorded on purpose:** the tracked contact now enters 3D NEARER than the FLEET does (`MODEL_ALT_CEIL_M` = 800,000 m, unchanged), so with the DISPLAY-rail `3D` toggle on, 150–800 km draws surrounding contacts as models while the selected one is still a glyph. Nothing double-draws (the fleet pass skips the tracked icao) and aligning the two is a fleet-side decision, deliberately out of scope. **The two thresholds are asymmetric on purpose:** a single threshold makes a tracked orbit sitting ON the boundary strobe billboard↔model as the camera's altitude wobbles across it. Do not collapse them. The latch is scoped per selection, so a new target re-evaluates against the ENTER ceiling rather than inheriting the previous target's exit band. Exactly ONE model is involved; it loads on demand when the regime opens, is HIDDEN (not released) on regime exit so re-entry has no load gap, and is released by the existing teardown on deselect/re-track/destroy. Cockpit and TR-3B suppression are unchanged. **Two invariants around it:** (a) the hysteresis latch AND the load-failure latch are per-selection state cleared by `_resetTrackedSelectionState()` in the tracking lifecycle (deselect / re-track / cross-layer / init / destroy) — the predicate's icao-change guard is defence only, since it needs a drawn frame while nothing is selected and the render governor's idle mode does not promise one; (b) on-demand loading is bounded at 3 attempts per selection with a 1.5 s backoff and one console warning naming the asset — the driver runs every `scene.preUpdate`, so an unbounded catch means a missing GLB spins load→reject at frame rate. The billboard stays the visual throughout a failed load.
 - **Grounded 3D handoff is terrain-validity gated (2026-08-23).** A ready civilian or military glTF does not own the visual until `groundSnap` (`src/data/groundSnap.js`) can answer with a MEASURED photoreal-surface height. On success the layer writes `height + the model's measured belly offset` before revealing the model. Cache movement is measured on the WGS84 surface, not across altitude, so a stationary contact keeps its snap through poll-time vertical-datum changes. Model existence or GPU readiness alone never suppresses the billboard floor, and ordinary zoom/style/deselect transitions preserve a valid snap cache. **Two states, and the difference is the whole design.** COLD — nothing has ever resolved for this icao (tiles still streaming on first sight, sample failure, backoff after a first miss): there is no evidence of where the ground is, `heightFor` returns null, the model stays hidden and the depth-test-free 2D billboard remains opaque and floored. WARM — a snap resolved and then a >`MOVE_INVALIDATE_M` (50 m) taxi move stopped it answering directly: the measurement is DEMOTED to a bounded last-known rather than deleted, and it keeps answering while the resample is outstanding, so a taxiing aircraft does not pop 3D→2D→3D across a 2–30 s retry backoff. The bound is `HELD_SNAP_MAX_DRIFT_M` = 250 m from the spot the value was measured at, past which the hold is dropped rather than stretched and the contact is COLD again. It is spatial with no timer beside it (ground under a contact that has not moved does not change; what invalidates the value is the contact MOVING) and deliberately a quarter of the billboard chain's `HELD_FLOOR_MAX_DRIFT_KM` — that hold only ever RAISES a sprite, while a held snap IS the model's placement, so its error shows in both directions. A fresh sample releases the hold, and so does a ground flip (which already calls `forget`). **A loading model is HIDDEN, never zero-scaled:** admission sets `show = false` (Cesium's default is `true`, and an unplaced primitive would claim the visual at the identity matrix), and ownership is `ready && show` — Cesium 1.138's `Model.update` has no `show` guard, so hiding a primitive costs its load nothing.
+- **Ground-snap clock rollback recovery.** If the wall clock moves before the
+  sampling-window start, ground snap opens a new window while retaining the
+  normal four-samples-per-250-ms budget. Retry timestamps more than the maximum
+  30-second backoff ahead of the current clock are ignored. A smaller rollback
+  can retain a bounded retry delay of up to 30 seconds.
 - Commercial and military aircraft use the same high-level FLEET model regime: 2D billboards when zoomed out, optional glTF models when closer, controlled by the DISPLAY rail `3D` toggle and `Proximity` / `All` modes. Since 2026-08-16 (Hangar fleet) models are PER-CLASS: real CC-BY GLBs for light/bizjet/turboprop/widebody/helicopter/uav (`CLASS_MODEL_REAL` in `src/data/aircraftClass.js` — meters-baked, scale 1, per-model belly/radius; provenance in `public/models/README.md`), the shared `airplane.glb` for the remaining civilian classes, and the military layer maps weight classes (real GLBs / 747 heavies / `jet.glb` fastjets, per-model heading offsets, flat amber or emergency red). Textured civilians carry a HEAVY tint, not a light one: `MODEL_COLOR_BLEND_AMOUNT` is `0.94` in BOTH layers under Cesium's `ColorBlendMode.MIX`, so the class colour supplies 94% of the surface and the asset's own texture ~6%. The visual direction is clean light silhouettes with only a weak diffuse contribution from the approved textures, so liveries deliberately do NOT read. IR boost raises the blend to a full `1.0`. Under NVG/FLIR (map preset or Cockpit vision) models render unlit flat-white at full alpha and scene fog is disabled (fog otherwise blacks out distant models with the globe hidden); state restores on exit.
 - **Aircraft models skip Cesium's dynamic environment map (2026-10-24).** Flights, Military Flights and Local ADS-B pass `environmentMapOptions: AIRCRAFT_ENVIRONMENT_MAP_OPTIONS` (`{ enabled: false }`, `src/data/aircraftModelLighting.js`) to every drawn model. Otherwise each `Model` renders the sky around it and reads it back from the GPU (`readPixels`) when it is first placed, and again whenever it moves more than 1 km, the scene's atmosphere settings change, or (under sunlight dynamic lighting) an hour of scene time passes (`DynamicEnvironmentMapManager`, Cesium 1.138). With the class colour supplying 94% of the surface, the models are lit from Cesium's default spherical harmonics instead, with no specular environment map, so the sky's model-lighting cube map no longer reaches aircraft. `src/data/aircraftModelLighting.test.mjs` pins the option at every drawn model call; the two preload calls only warm the glTF cache and are never drawn.
 - The DISPLAY-rail `3D` toggle is the user-facing activation path for both aircraft layers. Their small approved GLBs build their render resources without Cesium's frame-spread job queue, preventing continuous Photorealistic 3D Tiles streaming from starving model readiness; model caps, tracking, camera, and fallback billboards are unchanged.