feat: Akiba Crossing — parametric Akihabara rebuild of the urban map

Complete new approach after the hand-placed layout shipped misaligned:
every element now derives from a street grid and its building's footprint,
so alignment is structural, not luck.

District structure (modeled on Akihabara's Chuo-dori / Denki-gai / JR
viaduct):
- 16m avenue flanked by continuous mid-rise rows; buildings defined ONLY
  by z-interval + floor count (4m quantized floors), packed edge-to-edge
- _building() derives everything from the footprint: parapet lips on the
  roof rim, inset glass storefront, sidewalk canopy, stacked per-floor
  sign bands, a solid vertical corner sign column with glowing segments,
  evenly spaced roof AC units, rooftop billboard on 4+ floor buildings
- Brick rail viaduct crosses the whole block on piers with an under-track
  vending arcade; sidewalk-aligned slide ramps with handrails reach the
  deck on both flanks
- Denki-gai alley cut through both rows (wallrun walls + fire escape
  platforms climbing per floor); red SEGA-style landmark + blue
  electronics building anchor the north corners
- Matched streetlight pairs every 16m with arms over the road; zebra
  crossings at the alley mouth and south block; spawns on sidewalks/deck
  facing the map center

Movement: viaduct high lane + roof-hop onto the 2-floor row, canopy lane
above each sidewalk, alley wallruns + fire escapes, grapple anchors on
lamp arms / sign columns / viaduct edge / billboards, slide ramps.

Acoustics: facades concrete, storefronts glass, canopies wood, viaduct
brick, signs/AC/vending/lights/escapes metal.

Renamed in the level selector: "Akiba Crossing". Verified via aerial and
street captures (aligned rows, clean canyon, viaduct framing); movement
11/11, smoke 0 failures, acoustics ALL PASSED.

Co-Authored-By: Claude Fable 5 <[email protected]>

scenes/maps/neon_alley/map_meta.cfg

@@ -1,5 +1,5 @@
 [map]
-name="Neon Alley"
+name="Akiba Crossing"
 scene_path="res://scenes/maps/neon_alley/neon_alley.tscn"
 color1=Color(1.0, 0.3, 0.55, 1.0)
 color2=Color(0.2, 0.15, 0.45, 1.0)