Fix custom font fallback rendering

[gta66666668]

Fixes custom text rendering for glyphs that are not present in the selected Meteor font, such as Chinese characters.

The previous custom font renderer pre-packed a fixed set of Latin, Greek, Cyrillic, and symbol ranges. Characters outside those ranges could not render even when the OS had a suitable font installed.

Changes:

• load an available system fallback font for common CJK font families
• let Font use multiple font buffers
• lazily pack missing glyphs into the texture atlas
• upload pending glyphs before GUI/HUD custom text rendering
• handle Unicode code points instead of only Java char values

Validation:

• ./gradlew.bat build --stacktrace passes

This PR is based on master and does not include the Minecraft 26.2 update branch.

[MukjepScarlet]

Why you even need a list here? Isn't this just an Optional<FontFace>

[gta66666668]

Changed. You are right: this path selects at most one fallback face, so returning a List<FontFace> made the API more general than the implementation actually was. getFallbackFont(FontFace) now returns Optional<FontFace>, and readFontBuffers consumes it directly with flatMap / ifPresent.

I also tightened the related rendering path while rebasing this PR onto the latest master:

• fallback selection skips the entire primary family, prefers the primary style, then Regular, and uses locale-stable name matching;
• primary/fallback font buffers are loaded once and reused by GUI/HUD font sizes instead of rereading large CJK font files for every height;
• width measurement now caches glyph metrics without rasterizing into (and consuming) the texture atlas;
• missing prepacked glyphs correctly fall through to the fallback font, while supplementary Unicode code points remain handled as code points rather than UTF-16 chars;
• partially filled initial atlases are accepted safely, missing/unpackable glyphs are cached, and one packing failure no longer disables all later glyphs;
• renderer replacement and font cleanup are now atomic/idempotent, with HUD font resets deferred until the current frame finishes.

Validated with ./gradlew.bat build --stacktrace --console=plain (BUILD SUCCESSFUL).