Fix missing CJK/Indic text on Linux with lazy font fallback

[nibbbl]

Resolve fallback fonts per-glyph from FontConfig on demand instead of a truncated static list. This is a general improvement over the prior system, which simply slurped up 40 sorted fonts from the OS, causing missing characters on systems with many fonts. The newer system should result in less fonts being loaded, which is a nice bonus.

Closes #5991

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[Copilot]

Pull request overview

This PR improves Linux text rendering by switching font fallback from a truncated, pre-enumerated FontConfig font list to an on-demand per-codepoint lookup, allowing CJK/Indic/Thai glyphs to resolve correctly even on systems with very large font inventories.

Changes:

• Introduces LLFontFallbackMatch and a new LLWindow::findFallbackFontForChar() API for lazy, OS-driven font fallback.
• Implements Linux FontConfig matching per missing glyph (including face index for font collections), while stubbing the API on Windows/macOS.
• Updates LLFontFreetype::addGlyph() to lazily discover/load/attach fallback faces when existing fallback fonts don’t cover a codepoint.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
indra/llwindow/llwindow.h	Adds LLFontFallbackMatch and declares findFallbackFontForChar() for OS font fallback queries.
indra/llwindow/llwindow.cpp	Implements platform dispatch for LLWindow::findFallbackFontForChar().
indra/llwindow/llwindowsdl.h	Declares SDL/Linux implementation entrypoint for per-codepoint fallback.
indra/llwindow/llwindowsdl.cpp	Replaces Linux static fallback list with lazy FontConfig per-glyph matching.
indra/llwindow/llwindowwin32.h	Declares Windows stub for per-codepoint fallback API.
indra/llwindow/llwindowwin32.cpp	Adds Windows placeholder implementation (returns empty match).
indra/llwindow/llwindowmacosx.h	Declares macOS stub for per-codepoint fallback API.
indra/llwindow/llwindowmacosx.cpp	Adds macOS placeholder implementation (returns empty match).
indra/llrender/llfontfreetype.h	Adds state to support lazy fallback discovery and tracks attempted codepoints.
indra/llrender/llfontfreetype.cpp	Lazily queries OS for missing glyph coverage and loads fallback faces as needed.

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[nibbbl]

Thanks for the review, @marchcat. I'll address the recommendations from Copilot and review the build failures later today.

[marchcat]

@nibbbl, thank you!
I'll check it once it's ready.

[nibbbl]

@marchcat I've patched up the PR based on suggested Copilot comments. It looks like the builds are failing due to an unrelated problem related to a private dependency being required even for open source builds. I've punched up an issue #5998

[akleshchev]

lua branch doesn't have the 5e38c19, which probably is tha cause of the issue. I cherry picked it, please rebase or update from lua's branch.

[nibbbl]

lua branch doesn't have the 5e38c19, which probably is tha cause of the issue. I cherry picked it, please rebase or update from lua's branch.

Done. Thanks.