Fix black mission briefing map textures

[isaac-ranger]

Summary

• enforce the documented opaque-alpha contract for gos_Texture_Solid textures populated through Lock() / Unlock()
• preserve source alpha for keyed and alpha texture formats
• document the root cause and completed mc2_02 runtime verification

Root cause

The briefing thumbnail is passed to textureFromMemory() as gos_Texture_Solid. That path creates an empty texture, locks it, copies the source BGRA pixels, and unlocks it. Unlike the file-backed and encoded-memory paths, it bypasses convertIfNecessary() / makeKindaSolid(), so legacy zero-alpha values from the otherwise healthy source TGA reached the OpenGL texture. Normal alpha blending then exposed the black destination behind the map quad.

gosTexture::Unlock() now forces alpha to 255 only for gos_Texture_Solid while performing its existing BGRA-to-RGBA conversion. Keyed and alpha formats keep their source alpha.

Validation

• the affected gameos target compiles and links successfully in the native Linux build
• the full native executable linked successfully using build-only compatibility settings outside this branch
• modeled conversion makes all 16,384 pixels in the affected 128x128 thumbnail opaque without changing RGB data
• keyed and alpha formats do not take the new branch
• live mc2_02 mission-briefing verification showed blue water, colored terrain/buildings, the normal red objective marker, and a textured objective model with no black map regions or black texture quads

Scope

This PR changes only GameOS/gameos/gameos_graphics.cpp and the existing briefing-map investigation devlog. It adds no assets, binaries, launchers, campaign overrides, or other test-harness files.