Roads overlay: one polyline per segment + viewport culling (fix jank)

The per-pass thickness split fragmented each segment into a run per
weight change, turning a ~1.4k-segment network into ~84k polylines and
janking the map. Draw one polyline per segment at its median-pass width
instead (verified on device: 1430 vs 84083). Add buildRoadSegments
(segment + bbox) and cullRoadsInto (draw only segments meeting the padded
view, redraw on moveend/zoomend); the recording overlay now renders on a
canvas pane instead of SVG, and showRoadsMap culls too. Thickness is now
per-segment (median) rather than per-point — a deliberate simplification.

src-tauri/Cargo.lock

@@ -139,7 +139,7 @@ checksum = "c08606f8c3cbf4ce6ec8e28fb0014a2c086708fe954eaa885384a6165172e7e8"
 
 [[package]]
 name = "baldav"
-version = "0.1.513"
+version = "0.1.515"
 dependencies = [
  "base64 0.22.1",
  "btleplug",

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "baldav"
-version = "0.1.513"
+version = "0.1.515"
 edition = "2021"
 
 [lib]

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://raw.githubusercontent.com/nicegui/tauri-v2/refs/heads/main/tooling/cli/schema.json",
   "productName": "BalDav",
-  "version": "0.1.513",
+  "version": "0.1.515",
   "identifier": "dev.baldav.app",
   "build": {
     "frontendDist": "../frontend"