feat: add row-level diff rendering

[domenkozar]

What It Does

Extract per-row rendering from Canvas so individual rows can be written and compared independently. The terminal now diffs against the previous canvas and only re-renders rows that changed, reducing flicker and redundant output in both fullscreen and inline modes.

In fullscreen mode, absolute cursor positioning targets only changed rows. In inline mode, relative cursor movement achieves the same. The clear_canvas path in fullscreen mode now preserves output above the canvas area.

I'd be happy to add more tests for regressions since it's a substantial change in the core logic of rendering.

Related Issues

#117

[codecov]

Codecov Report

❌ Patch coverage is 93.49931% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.79%. Comparing base (6581aa2) to head (5e6ca04).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/iocraft/src/terminal.rs	94.95%	8 Missing and 20 partials ⚠️
packages/iocraft/src/canvas.rs	88.95%	4 Missing and 14 partials ⚠️
packages/iocraft/src/render.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
- Coverage   88.81%   88.79%   -0.02%     
==========================================
  Files          34       34              
  Lines        5945     5946       +1     
  Branches     5945     5946       +1     
==========================================
  Hits         5280     5280              
+ Misses        550      540      -10     
- Partials      115      126      +11     

Files with missing lines	Coverage Δ
packages/iocraft/src/components/text.rs	98.87% <100.00%> (-0.09%)	⬇️
packages/iocraft/src/render.rs	93.08% <75.00%> (-0.42%)	⬇️
packages/iocraft/src/canvas.rs	94.50% <88.95%> (-0.67%)	⬇️
packages/iocraft/src/terminal.rs	87.10% <94.95%> (+11.43%)	⬆️

... and 25 files with indirect coverage changes

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[domenkozar]

We're releasing this part of devenv 2.0.6 and will let you know what folks say.

[ccbrown]

Thanks for the PR! It looks great at a glance, but I'll do my best to review this fully in the next few days.

[owtaylor]

Neat stuff! - it was pretty much exactly what I needed for a project that I'm working on. I gave it some testing and found some issues with inline mode:

• Adding lines at the bottom edge of the screen wasn't right - it tried to move the cursor down to add another line, but couldn't. Newlines needed to be added instead to force scrolling.
• In the case of an inline buffer larger than the screen (the render_loop duplicate rendering issue #118 case), we don't want to use the scrollback-buffer-clearing fallback if we can avoid it - clearing the scrollback buffer, and rewriting everything is very likely to cause flashing, so there's a huge win in being precise.

I've put fixes for these at https://github.com/owtaylor/iocraft/tree/row-level-diff-fixes - feel free to add them to your PR if they look useful.

[domenkozar]

Thanks @owtaylor I've pulled your fixes in!

[domenkozar]

Note that I've reverted this change in devenv for now as it doesn't fully work yet. I'll test the bug fixes in following days.

[pamelia]

looking forward to this!