navi-offroute: vectorize road_terminus_transitions (O2b, perf) (#51)

Replace the per-road-cell Python 8-neighbour scan with a single 3x3 binary
dilation of the off-network mask (scipy.ndimage), AND'd with the road/track
mask. `border_value=0` treats out-of-bounds neighbours as on-network, matching
the scalar version's OOB skip -- NOT np.roll, which would wrap the raster edges
and fabricate phantom neighbours. A road cell is never off-network itself, so
dilating with the centre included is equivalent to the loop's strict-neighbour
test; the surviving cell set and the 2 directed foot<->vehicle tuples per cell
are identical (only emission order differs; cap/kernel are order-independent).

Synthetic eyeball benchmark (1234x470, dense road block, worst case for the
loop's early-break): scalar 843 ms -> vector 9.3 ms; set-equal True. Targets the
~3.75s road_terminus stage of Route B's gather_transition_cells.

Adds test_road_terminus_dilation_no_wrap (np.roll regression guard); existing
test_road_terminus_transitions_pure_raster + gather/cost parity tests unchanged.

Co-authored-by: mj <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/services/navi_offroute/tests/test_offroute.py

@@ -960,6 +960,19 @@ def test_road_terminus_transitions_pure_raster():
         assert sorted(edges) == sorted([(f, v, 60.0), (v, f, 60.0)])
 
 
+def test_road_terminus_dilation_no_wrap():
+    """O2b safety net: the 3×3 dilation must NOT wrap the raster edges. The only road cell sits
+    at corner (0,0) with all real in-bounds neighbours on-network; the only off-network cell is
+    the opposite corner (4,4). With border_value=0 the corner road cell has no off-network
+    neighbour -> no termini. np.roll would wrap (4,4) into (0,0)'s neighbourhood and falsely fire."""
+    rows, cols = 5, 5
+    trail_grid = _p3np.full((rows, cols), 10, _p3np.uint8)  # all on-network (trail), non-road
+    trail_grid[0, 0] = 5                                    # the only road cell, at the corner
+    trail_grid[4, 4] = 0                                    # the only off-network cell, opposite corner
+    meta = _p3_meta(rows, cols)
+    assert _trans.road_terminus_transitions(meta, trail_grid) == []
+
+
 def test_transition_cap_closest_15(monkeypatch):
     """>15 parking lots within 5 km -> only the closest 15 (by perp distance) survive."""
     line = ((40.0, -111.0), (40.0, -110.0))           # ~east-west; lat offset = perp distance, all <5 km

backend/services/navi_offroute/transitions.py

@@ -10,6 +10,7 @@ DEMReader.latlon_to_pixel (shared/dem.py).
 import math
 
 import numpy as np
+import scipy.ndimage as ndi
 
 from .cost import (
     TRANSITION_COST_PARKING_S,
@@ -143,32 +144,28 @@ def trailhead_transitions_near_line(line, buffer_m=5000):
 def road_terminus_transitions(meta, trail_grid, elevation=None):
     """Road-terminus mode switches from the trail raster (spec §4): a road(5)/track(15) cell
     with a passable off-network 8-neighbour (value 0; finite elev when `elevation` given).
-    foot↔vehicle at TRANSITION_COST_ROAD_TERMINUS_S. Pure raster scan, no DB — fixes §1."""
-    rows, cols = trail_grid.shape
+    foot↔vehicle at TRANSITION_COST_ROAD_TERMINUS_S. Pure raster scan, no DB — fixes §1.
+
+    O2b: the per-road-cell 8-neighbour Python loop is replaced by one 3×3 binary dilation of
+    the off-network mask (scipy.ndimage). `border_value=0` treats out-of-bounds neighbours as
+    on-network, matching the scalar version's OOB skip — NOT np.roll, which would wrap the
+    raster edges and fabricate phantom neighbours. A road cell is never off-network itself, so
+    dilating with the centre included is equivalent to the loop's strict-neighbour test. The
+    surviving cell set (and the 2 directed tuples per cell) is identical; only emission order
+    differs, and the cap / kernel consume the cells order-independently."""
     pairs = (("foot", "vehicle"),)
     road = (trail_grid == 5) | (trail_grid == 15)
-    rs, cs = np.nonzero(road)
+    offnet = (trail_grid == 0)
+    if elevation is not None:
+        offnet &= np.isfinite(elevation)
+    offnet_neighbour = ndi.binary_dilation(
+        offnet, structure=np.ones((3, 3), dtype=bool), border_value=0)
+    terminus = road & offnet_neighbour                # road cell with ≥1 off-network 8-neighbour
+    rs, cs = np.nonzero(terminus)
     out = []
     for r, c in zip(rs.tolist(), cs.tolist()):
-        is_terminus = False
-        for dr in (-1, 0, 1):
-            for dc in (-1, 0, 1):
-                if dr == 0 and dc == 0:
-                    continue
-                nr, nc = r + dr, c + dc
-                if nr < 0 or nr >= rows or nc < 0 or nc >= cols:
-                    continue
-                if trail_grid[nr, nc] != 0:
-                    continue
-                if elevation is not None and not np.isfinite(elevation[nr, nc]):
-                    continue
-                is_terminus = True
-                break
-            if is_terminus:
-                break
-        if is_terminus:
-            lat, lon = _pixel_to_latlon(r, c, meta)
-            out.extend(_bidir(lat, lon, pairs, TRANSITION_COST_ROAD_TERMINUS_S))
+        lat, lon = _pixel_to_latlon(r, c, meta)
+        out.extend(_bidir(lat, lon, pairs, TRANSITION_COST_ROAD_TERMINUS_S))
     return out