Fix GitHub Pages Freedoom web build

[sabino]

Summary

• build a separate Freedoom-based web ROM artifact for GitHub Pages
• keep the native ROM artifact unchanged while using 2 MiB C-ROM chips for FBNeo web compatibility
• make the Pages packager reject oversized/mismatched FBNeo chip sizes with a clear error
• document the Pages/native ROM split

Verification

• python3 -m py_compile tools/doomgeo_build.py tools/doomgeo_plan.py tools/gen_gfx.py tools/doom_convert.py
• python3 tools/doomgeo_build.py pages --rom build/pages-freedoom-test-rom/puzzledp.zip --bios build/pages-freedoom-test-rom/neogeo.zip --asm-rom build/asm-rom/puzzledp.zip --out build/pages-freedoom-site-test
• verified generated web ROM, ASM ROM, and BIOS zip entry sizes/CRCs
• verified the Pages packager rejects the current native 4 MiB C-ROM artifact with the new explicit error