Cache font metrics during redraw to reduce Unicode TUI stalls

[Cianidos]

Summary

This reduces redraw stalls for Unicode-heavy TUIs such as btop by avoiding repeated query-font calls for the same font during a single Ghostel redraw.

Fixes #291

Changes

• Add ghostel--query-font-cached, dynamically backed by a redraw-local hash table.
• Bind the cache around the native renderer call in ghostel--delayed-redraw.
• Have the native renderer call ghostel--query-font-cached instead of query-font.
• Dynamically raise gc-cons-threshold and gc-cons-percentage only while rendering a frame.
• Add a focused test for the query-font cache.

Why

Profiling a btop session showed redraw spending significant time in repeated Emacs font metric queries and automatic GC. A single redraw could call query-font thousands of times for repeated fonts.

This patch keeps existing glyph adjustment behavior intact. It intentionally does not skip font-at or font-get-glyphs, because an attempted default-font fast path caused visible TUI border artifacts.

Validation

• zig fmt src/Renderer.zig src/emacs.zig
• make .build/tests/elisp-ghostel-glyph-kitty-test.ok
• make .build/tests/native-ghostel-glyph-kitty-test.ok

Manual validation:

• btop at 100ms update interval remains responsive.
• No shifted border or visual artifacts observed after removing the unsafe glyph-adjustment skip.

[emil-e]

Thanks for the PR. This is a good improvement and I think we should merge it.

Long term, I think it's even better to cache the scaling per glyph but I can take it upon me to implement that, this at least improves the immediate case.

[emil-e]

LGTM. @dakra Any concerns?

[dakra]

lgtm. I'll simplify the gc-cons-threshold code a bit and then merge.
Don't think we need a defcustom for it and can just use the let* binding we already have for the whole redraw function.
The Emacs native pixel-scroll.el does also just

          ;; The animations are smoother if the GC threshold is
          ;; reduced for the duration of the animation.
          (gc-cons-threshold (min most-positive-fixnum
                                  (* gc-cons-threshold 3)))

to temproarily disable garbage collection. So think the same is fine for us and we don't need 2 more defcustoms.

[dakra]

Merged. Thanks ❤️