props: add RustChain vintage PowerPC tower

pk3_build/models/props/rustchain_vintage_powerpc_tower/LICENSE

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+RustChain Vintage PowerPC Tower prop
+
+Copyright 2026
+
+This model, texture pack, preview image, and deterministic procedural source
+are licensed under the Creative Commons Attribution-ShareAlike 4.0
+International License (CC-BY-SA-4.0).
+
+You are free to share and adapt the material for any purpose, even
+commercially, provided attribution is given and adaptations are distributed
+under the same license.
+
+License summary and legal code:
+https://creativecommons.org/licenses/by-sa/4.0/

pk3_build/models/props/rustchain_vintage_powerpc_tower/README.md

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+# RustChain Vintage PowerPC Tower prop
+
+Original low-poly set-dressing prop for the Xonotic RustChain Arena prop bounty #14015:
+https://github.com/Scottcjn/rustchain-bounties/issues/14015
+
+## Package
+
+| File | Purpose |
+| --- | --- |
+| `rustchain_vintage_powerpc_tower.iqm` | Static IQM model for Xonotic/DarkPlaces |
+| `rustchain_vintage_powerpc_tower.obj` | Editable source mesh generated by the Python script |
+| `rustchain_vintage_powerpc_tower.mtl` | Source material table for the OBJ |
+| `rustchain_vintage_powerpc_tower.skin` | Xonotic material-to-texture mapping |
+| `textures/*.tga` | 256x256 power-of-two texture set |
+| `preview.png` | Generated 1280x720 preview sheet |
+| `README.md` | Integration and design notes |
+| `LICENSE` | CC-BY-SA-4.0 / GPL-compatible asset license |
+
+## Design
+
+This implements the **Vintage PowerPC / retro tower** wishlist item. The prop is
+a beige late-1990s tower silhouette with stacked optical/floppy bays, vertical
+front vents, a small cyan RTC activity window, colored side badge stripes, rear
+port hints, and chunky pedestal feet. It is meant for antiquity-vault corners,
+server rooms, spawn alcoves, and map-side storytelling around old silicon.
+
+The geometry and textures are original procedural assets. No external mesh,
+photo texture, AI-generated model file, or trademarked logo is embedded.
+
+## Regeneration
+
+From the repository root:
+
+```powershell
+python tools/generate_rustchain_vintage_powerpc_tower_prop.py
+```
+
+The script writes the OBJ/MTL source, TGA textures, PNG preview, README,
+LICENSE, and skin file. The committed IQM was converted from the generated OBJ
+with the repository IQM compiler rebuilt locally for the current Linux runtime:
+
+```bash
+cd source/iqm
+g++ -O3 -fomit-frame-pointer -Wall -fsigned-char -o /tmp/iqm-local iqm.cpp
+cd ../..
+/tmp/iqm-local pk3_build/models/props/rustchain_vintage_powerpc_tower/rustchain_vintage_powerpc_tower.iqm   pk3_build/models/props/rustchain_vintage_powerpc_tower/rustchain_vintage_powerpc_tower.obj
+```
+
+## Placement
+
+Suggested use: low-cover set dressing against walls, behind glass, or inside an
+antiquity vault. The model is static and low-poly; mappers can scale it to fit
+server-room corridors or museum-style displays.

pk3_build/models/props/rustchain_vintage_powerpc_tower/rustchain_vintage_powerpc_tower.mtl

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+# Material table for rustchain_vintage_powerpc_tower
+newmtl powerpc_case
+Ka 0.000 0.000 0.000
+Kd 0.720 0.680 0.580
+Ks 0.050 0.050 0.050
+Ns 16.0
+map_Kd textures/powerpc_case.tga
+
+newmtl powerpc_dark
+Ka 0.000 0.000 0.000
+Kd 0.120 0.130 0.140
+Ks 0.050 0.050 0.050
+Ns 16.0
+map_Kd textures/powerpc_dark.tga
+
+newmtl powerpc_glow
+Ka 0.000 0.000 0.000
+Kd 0.050 0.840 0.880
+Ks 0.050 0.050 0.050
+Ns 16.0
+map_Kd textures/powerpc_glow.tga
+
+newmtl powerpc_label
+Ka 0.000 0.000 0.000
+Kd 0.820 0.500 0.180
+Ks 0.050 0.050 0.050
+Ns 16.0
+map_Kd textures/powerpc_label.tga