fix: render on Ghostty's physical pixel grid

[Hona]

Summary

• rebuild canvas and font metrics when browser DPR changes instead of scaling a stale backing store
• derive CSS dimensions from integer physical cells and paint contiguous backgrounds as exact physical-pixel runs
• expose Ghostty core's real font.sprite.Face rasterizer through the WASM API
• render Ghostty-native block, box, braille, Powerline, branch, geometric, and legacy-computing sprites instead of font approximations
• snap decorations and cursors to Ghostty-derived integer physical metrics

Source of truth

The implementation follows the pinned Ghostty core:

• src/font/Metrics.zig: physical cell width/height round to nearest integer
• src/renderer/shaders/glsl/cell_bg.f.glsl: backgrounds resolve from physical pixel to cell
• src/font/sprite/Face.zig: canonical sprite range dispatch and rasterization
• src/font/sprite/draw/*: canonical Ghostty geometry

No glyph geometry is copied from xterm.js or Windows Terminal. TypeScript only handles browser DPR, tinting, caching, and compositing of Ghostty-produced A8 masks.

Visual validation

A Playwright-driven Electron harness initialized at 100%, changed zoom without reload, and captured old/reloaded/fixed output at 80%, 125%, and 150%.

• old live zoom retained a 1.00 backing ratio
• old reload rebuilt at the target ratio
• fixed live zoom rebuilt immediately at 0.80, 1.25, and 1.50
• fixed solid backgrounds have no internal cell outlines
• fixed Powerline and block glyphs use Ghostty core masks

Verification

• bun test (365 passed)
• bun run typecheck
• targeted Biome checks
• production library build
• Zig 0.15.2 WASM build
• patch applicability and clean submodule verification

The native sprite support increases ghostty-vt.wasm from 639,033 to 967,164 bytes.

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