Rebases goteusz-maszyk's PR #882 onto current main, keeping main's
terrain-flattening architecture (perpendicular median, row_medians
cache, direction-aware curb outlines, road-surface-Y registration)
while integrating the PR's user-visible improvements:

- New `surfaces` module with `get_blocks_for_surface` /
  `get_blocks_for_surface_way` / `semirandom_surface`, shared by
  highways, leisure, and railways (platform=*).
- OSM `surface=*` tag now drives the road block mix (was only applied
  to pedestrian plazas before).
- OSM `width=*` tag now overrides the default block-range for a way
  (clamped to the terrain helper's sample cap).
- `lanes=*` drives inner lane-divider count; `lane_markings=no`
  disables them. Dividers are drawn per centerline bresenham point
  with a proper dash/gap state machine (fixing the solid-line
  regression in an earlier PR iteration) and their Y still rides the
  perpendicular-median-flat cross section.
- `indoor=yes` plus normalised `layer=*` are hoisted to the top of
  the highway function so street_lamp / bus_stop now rise with their
  overpass instead of sitting at ground level.
- Flat-roofed outlines stay as main's direction-aware
  `x ± (block_range+1)` / `z ± (block_range+1)` curbs — the PR's
  four-sided box outline overwrote road edges and is dropped.
- Unused `set_block_flag_absolute` helper removed; the `set_block`
  → `set_block_absolute` delegation from the PR is kept.

Squashes the PR's four commits because its follow-up fixes depended
on the initial refactor which needed heavy rework against main's
new terrain-flattening pipeline.

Co-authored-by: goteusz-maszyk <goteusz-maszyk@users.noreply.github.com>

Rebase #882: unify surfaces and add multi-lane markings onto main