feat(display/collapsible-buffer): introduce reusable CollapsibleBuffer for thinking streams and future reuse

[cto-new]

Summary

Introduce a reusable collapsible/scrollable buffer to standardize thinking output and future buffers.

Details

• Added CollapsibleBuffer in paws_common and wired export via display module.
• Implemented timing, max-height truncation, and chunked writing with optional dimming.
• Integrated CollapsibleBuffer into paws_main UI to handle thinking output instead of ad-hoc Markdown buffering.
• Added unit tests for CollapsibleBuffer covering initialization, timing, reset, and dimming behavior.
• This enables consistent incremental rendering and easier reuse for future content.

Warning: Task VM test is not passing, cto.new will perform much better if you fix the setup

[manthanabc]

Note to self:
This approach makes little sense as it is, unless a way can be found to render multiple of such buffers while having a sane way to manage cursor location.
Maybe a better approach would be to elm like a model which can always be converted to a visual representation, while also avoiding to ever re rendering the text for converting state to ui on every update. Since the app already feels a bit flikery on windows..