Implement map-specific postcard textures

[SnipUndercover]

Closes #1043. Allows mappers to specify the texture of the postcard in the map's .meta.yaml, like so:

# relative to the Gui atlas
PostcardTexture: "SnipUndercover/EverestDevelopment/postcard"

If the texture does not exist, the __fallback texture will be displayed. Because the fallback texture is so small, most of the text will be cut off by it.

I suppose a next PR could let mappers change the color of the savedata name text that's rendered on the postcard to allow dark postcards?

[SnipUndercover]

It seems that GH and Azure can't agree on the commit hash. I'll push an empty commit.

[SnipUndercover]

That jogged it just right, the TAS check is running.

[Wartori54]

Looks good.
Together with allowing text color to be changed it would be nice to also add text repositioning in that upcoming PR for a more complete feature.

[SnipUndercover]

Will create an issue and assign myself to it to not forget.

[maddie480-bot]

The pull request was approved and entered the 3-day last-call window.
If no further reviews happen, it will end on Jan 6, 2026, 8:00 PM UTC, after which the pull request will be able to be merged.

[maddie480-bot]

The last-call window for this pull request ended. It can now be merged if no blockers were brought up.

[SnipUndercover]

This will need a slight change in the .meta.yaml format to prepare for a future PR; do not merge yet.

[maddie480-bot]

The last-call window for this pull request ended. It can now be merged if no blockers were brought up.

[microlith57]

marked as draft at @SnipUndercover's request