fix(render): clip symbols to terminal cells

[cpendery (cpendery)]

Summary

• snap Powerline transforms to the same integer pixel bounds as terminal backgrounds and cursors
• clip private-use and block-element outlines to their owning grid cell with a reusable renderer mask
• fit semantic full-cell and shade glyphs to all four grid edges, matching browser cell geometry while preserving title font metrics
• cover Powerline, Nerd Font, full-block, shade, and window-title glyphs at 1.25x and 2x raster scales
• update the deterministic Nerd Font GIF/PNG render snapshots

Validation

• cargo clippy -p tui-test-rs --all-targets --all-features -- -D warnings
• cargo clippy -p tui-test-rs --all-targets --no-default-features -- -D warnings
• cargo test -p tui-test-rs --all-features -- --test-threads=1
• bash scripts/regenerate-static-media.sh (all generated static timelines were pixel-identical, so timing-only churn was discarded)

Before and after

The images are 4x nearest-neighbor crops of the same deterministic snapshot. On main, the █▓▒░ row begins five source pixels inside the Powerline row and antialiased symbol pixels can escape neighboring cells. Afterward, symbols stop at the shared snapped edge, and █▓▒░ each fill the complete cell like the web rendering. The regression measured 200 gray bleed pixels before and 0 after and requires every semantic full-cell glyph to touch all four boundaries.

Before	After