Cap raster resolution to GPU MAX_TEXTURE_SIZE

[mucow24]

Summary

• At extreme zoom (~13× and up on a 16384 GPU at DPR 1.5), Pixi's text textures and filter render targets exceeded gl.MAX_TEXTURE_SIZE. The failed allocation composited a black/garbage rect over the affected region — visually like a near-plane camera clipping through high-z items, with lower layers (canvas surface, shapes) only visible where no text would have covered them.
• New helper clampRasterResolution(requested, itemDim, maxTextureSize) in rasterCaps.ts (with cached GL probe getMaxTextureSize()).
• pixiText.resolution, ClampingBlurFilter.resolution, and DropShadowFilter.resolution are clamped against the cap. Past the threshold, glyphs/filters go soft instead of disappearing.
• The image-adjustment path already had a flat Math.min(2, …) cap with the same root cause in its comment ("clipping shows up as rectangular edges on filtered output") — left unchanged here, but the new helper would generalize it.

Test plan

• npm run preflight (lint, typecheck, unit, e2e) — all green
• Local manual: zoom past 1300% on a multi-text doc, canvas no longer goes black
• Verify on a doc with blurRadius > 0 items at extreme zoom
• Verify on a doc with drop-shadowed shapes at extreme zoom

🤖 Generated with Claude Code