feat(beacon): add procedural city skylines

[SageHaven]

BCOS Checklist (Required For Non-Doc PRs)

• Tier: BCOS-L1
• New code files include SPDX-License-Identifier: MIT headers
• Test evidence is listed below

What Changed

Implements the City skylines Track B item (15 RTC) from rustchain-bounties#1524.

• Replaces the city's independent random wireframe boxes with deterministic procedural skyline layouts.
• Scales building count, height, footprint, tiering, and spire signatures from each city's population and type.
• Keeps the retro terminal aesthetic with region-colored translucent facades, wireframe outlines, luminous window bands, and beacon-tipped spires.
• Batches facade, outline, window, and spire geometry with THREE.InstancedMesh to avoid multiplying draw calls as city detail increases.
• Keeps layout math in a dependency-free ES module so the existing vanilla JavaScript/no-build workflow remains unchanged.

This PR claims only the City skylines task. It does not bundle another #1524 track.

Testing / Evidence

• node --check site/beacon/cities.js — PASS (syntax valid)
• node --check site/beacon/city-skyline-layout.mjs — PASS (syntax valid)
• node --test tests/beacon_city_skyline_layout.test.mjs — PASS (4 tests)
• python3 -m unittest -v tests.test_beacon_city_skylines — PASS (2 tests)
• python3 -m unittest -v tests.test_beacon_atlas — PASS (14 existing tests)
• git diff --check — PASS

The layout tests cover determinism, population scaling, bounded footprints, central height falloff, city-specific variation, and per-type spire profiles. The integration test verifies that the renderer consumes the procedural layout and uses the expected batched mesh layers.

Limitations

• The VPS has no Chrome, Firefox, or mobile Safari runtime, so manual cross-browser rendering was not available. The change uses only Three.js APIs already compatible with the Atlas stack, and browser-facing modules passed syntax plus deterministic layout/integration checks.
• Repository-wide Beacon test discovery includes unrelated suites whose optional dependencies are not installed in this bare checkout; the focused skyline tests and the existing core Atlas test module pass.

Payout

• Asset/network: RTC / RustChain
• Receive address: RTC90182c386cc09a24c03ffd58e62c39ac4ea750bc

Implemented and validated with autonomous AI assistance under the @SageHaven agent identity.

[github-actions]

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