Extract shared RoutePolylineReconciler for both map flavors (#1906)

[bmander]

Fixes #1906.

Problem

The route-polyline reconcile/width algorithm (~60 lines) was duplicated verbatim across GoogleMapRenderer and MapLibreRenderer: the identity/equality early-exit, the reconcileEqualItems diff, per-index width comparison against the stored widths, the native width patch, the three-field state update, and the camera-settle width resync. As the issue notes, a single bug fix had recently had to be applied by hand to both copies — the maintenance hazard this refactor removes.

Change

Introduce RoutePolylineReconciler<NativeLine> in the existing map/render module. It owns all the flavor-neutral bookkeeping and is parameterized by the four genuinely platform-specific operations over the opaque native-line type:

• widthOf(polyline, zoom) — resolve a line's pixel width (gms scales by density; maplibre doesn't)
• createLine(polyline, width) — mint a native line
• removeLines(lines) — batch removal (gms remove() per line; maplibre removeAnnotations)
• setWidth(line, width) — patch a live line's width

Each renderer now holds a RoutePolylineReconciler<Polyline> supplying those, and renderRoutePolylines / onCameraSettled / dispose collapse to single delegations (reconcile / resyncWidths / clear). Three private state fields and the reconcileEqualItems import drop out of each renderer.

Behavior is unchanged — a pure extraction. The MapLibre dispose keeps its exact prior semantics (route lines cleared before the mass removeAnnotations()).

Tests

New RoutePolylineReconcilerTest (11 cases) drives the shared logic through a fake native line: retain-equal, remove-gone, create-new, patch-changed-width, patch-skipped-when-unchanged, the same-list O(1) no-op, resyncWidths, and clear.

Verification

• Both flavors compile clean under -PwarningsAsErrors=true (the CI gate).
• Render unit-test suite passes on obaGoogle and obaMaplibre.
• Rebased onto latest main (Cleanup: small-exhaust sweep from the 2026-07-16 debt review #1915); main's concurrent renderer edits were comment/constant drift outside the reconcile path, so the merge was clean.

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