Read the mesh floor where transit vehicles and grounded flights actually are

With readings kept per point, the floor a vehicle gets depends on which point
is probed and where it is read. Transit offered the sampler the first point of
each vehicle's path in a cell, usually its oldest fix there, so a vehicle that
stayed in one cell never had its current spot probed. It now offers the latest
point of the path in each cell, and the newest fix in the vehicle's own cell
(the selected vehicle's history is walked newest first).

The grounded-flight display clamp read the floor at its sticky cell's centre,
which now answers from the reading nearest the centre, not the contact. It
reads at the contact's own position, held inside the sticky cell so boundary
hysteresis still applies.

docs/CURRENT-STATE.md

@@ -1360,13 +1360,16 @@ the same way; a cell keeps at most sixteen). A read answers from the cell's
 reading nearest the asked point, so a viaduct and the street beside it in one
 cell keep their own heights; two readings within about 5 m of each other keep
 the lower. A spot refused only because the cell's DEM prior was cold is tried
-again later. Before this, one probe at the cell's centre latched for the
-session, so a centre on a roof lifted everything in the cell (a Haymarket bus
-drew 23.5 m above the street, on the garage roof) and a centre in the river put
-Red Line trains on the Longfellow Bridge 6–9 m under the deck. Residual: the
-probe is vertical, so a vehicle directly under a deck or a roof overhang reads
-the structure above it, and a vehicle between readings takes the nearest one, so
-its floor steps where the nearest reading changes.
+again later. Transit offers the sampler the latest point of each vehicle's path
+in a cell (its newest fix in its own cell), and the grounded-flight display
+clamp reads at the contact, held inside its sticky cell. Before this, one probe
+at the cell's centre latched for the session, so a centre on a roof lifted
+everything in the cell (a Haymarket bus drew 23.5 m above the street, on the
+garage roof) and a centre in the river put Red Line trains on the Longfellow
+Bridge 6–9 m under the deck. Residual: the probe is vertical, so a vehicle
+directly under a deck or a roof overhang reads the structure above it, and a
+vehicle between readings takes the nearest one, so its floor steps where the
+nearest reading changes.
 
 ## Vessel components and sources