fix: Add periodic Kitty image garbage collection during scroll

[kaantradelab]

Problem:
When using Kitty graphics protocol, orphan images accumulate in kittyGraphicsState.imagesById because cleanupUnusedKittyImages() is only called on explicit delete commands.

During long-running terminal sessions with heavy output (e.g., CI/CD logs, AI agent sessions), this causes memory to grow unbounded. Observed: 11GB+ memory usage, terminal freeze, inability to scroll.

Solution:

• Add counter-based garbage collection in scroll()
• Track lines scrolled since last cleanup
• Trigger cleanupUnusedKittyImages() every 500 lines when buffer is trimmed
• Change function visibility from private to internal

Results:

• Memory usage reduced from 11GB+ to ~400MB (96%+ improvement)
• Terminal remains responsive during long sessions

[migueldeicaza]

Oops, thank you so very much for tracking this!

Let me see if I can do something a little more aggressive, rather than doing a cycle scan every N scrolled lines, if that does not pan out, I will merge your PR.

[migueldeicaza]

Btw, the reason I do not want to do a collection after the number of lines is both that it would add another layer of uncertainty and jumps while I am profiling the performance and because it would probably break the kitty protocol.

Since reference counting does not work (as this is part of the design), what I am going to do is add an eviction of data after certain size.

[migueldeicaza]

Ok, this is what I am doing instead:

#433