Improve flterm resize stability and cursor rendering

[dux-web]

Summary

• keep the render box aligned to the bounded viewport while deriving terminal rows and columns from whole cells
• avoid redundant resize callbacks when viewport changes do not alter the terminal grid
• add a configurable bar cursor width ratio and keep hollow cursor strokes inside their cells

Motivation

Sizing the render box to the whole-cell grid caused its visual origin to shift by a few pixels as the viewport crossed fractional-cell widths. Filling the viewport while retaining whole-cell grid dimensions keeps terminal content stable without debouncing or replaying frames.

Testing

• dart format --set-exit-if-changed packages/flterm
• dart analyze packages/flterm
• dart pub publish --dry-run (0 warnings)
• flutter test --exclude-tags golden (651 tests passed on macOS arm64 with Zig 0.16.0 and the pinned Ghostty source)