Document required base language archives

[themuffinator]

Summary

• explain that the unsuffixed retail zpak_<language>.pk4 archive is required alongside numbered language patches
• warn that missing dialogue media can also stop conversation-gated campaign progression
• record the completed reporter-log investigation and controlled airdefense2 A/B in the release-completion evidence

Investigation evidence

• issue Linux x86_64: character dialogue audio is missing #87 log SHA-256: 378A7FD5B9FFE969F49EA335C8E346FD0BF26CBEB5B98F9F0DAA18B41A2839EE
• issue Single-player scripted NPC movement stalls progression #88 log SHA-256: 316B12166FBDBDC64965143131550D0B716147411C225F4ACB7625C41F65794B
• both logs mount only zpak_english_01.pk4 through _04 and print the missing-base-archive warning
• with that incomplete media set, the stock airdefense2 conversation remains before Bidwell's movement and he stays near (5372, 2625, 244) through 75 seconds
• with the unsuffixed archive mounted, the same current-source sequence reaches (5437, 2575, 244) by 60 seconds with no navigation failure

Validation

• git diff --check
• python tools/tests/linux_appimage_packaging.py
• python tools/tests/release_tooling_safety.py

[themuffinator]

CI note: the four red macOS Push Verification jobs are an inherited hosted-runner renderer-smoke limitation, not a documentation regression. Their reports expose only an OpenGL 2 compatibility context, reject GLSL 1.30, and stop in the same FBO capability diagnostics as the four corresponding jobs on current main (baseline run 33019666992).

The PR's Windows, Linux x64, Linux ARM64, validation-smoke, ARM64 cross-build, and all thin-architecture macOS Commit Validation jobs have passed. The sanitizer and universal2 assembly jobs are still finishing; I’ll wait for those before merging.

[themuffinator]

Final CI disposition: both universal2 assembly jobs passed. The only unfinished check is Linux x64 ASan+UBSan Commit Validation, which has remained inside Run sanitizer build since 23:04 UTC without a step transition. This branch changes only Markdown; the identical engine/GameLib source already passed that sanitizer job in the immediately preceding successful commit-validation run (33015438667). Together with the passing Windows, Linux, ARM64, Wayland, thin-macOS, universal2, cross-build, and script checks here, that establishes the change is safe to merge despite the stalled runner.