tui: only check for emojis in visibleWidth when necessary

The initial render of a session, and any re-draws caused by terminal
resizing are noticeably slow, especially on conversations with 20+
turns and many tool calls.

From profiling with `bun --cpu-prof` (available since bun 1.3.2), the
majority of the rendering (90%) is spent on detection of emojis in the
string-width library, running the expensive `/\p{RGI_Emoji}$/v`
regular expression on every individual grapheme cluster in the entire
scrollback. I believe it essentially expands to a fixed search against
every possible emoji sequence, hence the amount of CPU time spent in it.

This change replaces the `stringWidth` from string-width with a
`graphemeWidth` function that performs a similar check, but avoids
running the `/\p{RGI_Emoji}$/v` regex for emoji detection unless it
contains codepoints that could be emojis.

The `visibleWidth` function also has two more optimisations:
- Short-circuits string length detection for strings that are entirely
  printable ASCII characters
- Adds a cache for non-ASCII segments to avoid recomputing string length
  when resizing

packages/tui/CHANGELOG.md

@@ -12,11 +12,13 @@
 ### Changed
 
 - README.md completely rewritten with accurate component documentation, theme interfaces, and examples
+- `visibleWidth()` reimplemented with grapheme-based width calculation, 10x faster on Bun and ~15% faster on Node ([#369](https://github.com/badlogic/pi-mono/pull/369) by [@nathyong](https://github.com/nathyong))
 
 ### Fixed
 
 - Markdown component now renders HTML tags as plain text instead of silently dropping them ([#359](https://github.com/badlogic/pi-mono/issues/359))
 - Crash in `visibleWidth()` and grapheme iteration when encountering undefined code points ([#372](https://github.com/badlogic/pi-mono/pull/372) by [@HACKE-RC](https://github.com/HACKE-RC))
+- ZWJ emoji sequences (rainbow flag, family, etc.) now render with correct width instead of being split into multiple characters ([#369](https://github.com/badlogic/pi-mono/pull/369) by [@nathyong](https://github.com/nathyong))
 
 ## [0.29.0] - 2025-12-25