fix(tui): don't swallow Kimi/Qwen ~! ~? kaomoji as subscript spans

The inline markdown regex had `~([^~\s][^~]*?)~` for Pandoc-style subscript
(H~2~O, CO~2~). On models that decorate prose with kaomoji like `thing ~!`
and `cool ~?` — Kimi especially — the opener `~!` paired with the next
stray `~` on the line and dim-formatted everything between them with a
leading `_` character, mangling markdown output.

Tighten the pattern to short alphanumeric-only content (`~[A-Za-z0-9]{1,8}~`)
since real subscript never contains punctuation, spaces, or long runs.
Same tightening applied to stripInlineMarkup so width measurement stays
consistent. Classic CLI was unaffected because it renders these literally.

ui-tui/src/__tests__/markdown.test.ts

@@ -23,6 +23,31 @@ describe('INLINE_RE emphasis', () => {
     expect(matches('a*b*c')).toEqual(['*b*'])
     expect(matches('a**bold**c')).toEqual(['**bold**'])
   })
+
+  it('matches short alphanumeric subscript (H~2~O, CO~2~, X~n~)', () => {
+    expect(matches('H~2~O')).toEqual(['~2~'])
+    expect(matches('CO~2~ levels')).toEqual(['~2~'])
+    expect(matches('the X~n~ term')).toEqual(['~n~'])
+  })
+
+  it('ignores kaomoji-style ~! and ~? punctuation', () => {
+    // Kimi / Qwen / GLM emit these as decorators and the whole span between
+    // two tildes used to get collapsed into one dim blob.
+    expect(matches('Aww ~! Building step by step, I love it ~!')).toEqual([])
+    expect(matches('cool ~? yeah ~?')).toEqual([])
+    expect(matches('mixed ~! and ~? flow')).toEqual([])
+  })
+
+  it('ignores tilde spans that contain spaces or punctuation', () => {
+    // Real subscript doesn't contain spaces; a tilde followed by words-then-
+    // tilde is almost always conversational. Matching it swallows text.
+    expect(matches('hello ~good idea~ there')).toEqual([])
+    expect(matches('x ~oh no!~ y')).toEqual([])
+  })
+
+  it('does not let strikethrough eat subscript', () => {
+    expect(matches('~~strike~~ and H~2~O')).toEqual(['~~strike~~', '~2~'])
+  })
 })
 
 describe('stripInlineMarkup', () => {
@@ -31,6 +56,11 @@ describe('stripInlineMarkup', () => {
     expect(stripInlineMarkup('browser_screenshot_ecc.png')).toBe('browser_screenshot_ecc.png')
     expect(stripInlineMarkup('__bold__ and foo__bar__')).toBe('bold and foo__bar__')
   })
+
+  it('leaves ~!/~? kaomoji alone and still handles real subscript', () => {
+    expect(stripInlineMarkup('Yay ~! nice work ~!')).toBe('Yay ~! nice work ~!')
+    expect(stripInlineMarkup('H~2~O and CO~2~')).toBe('H_2O and CO_2')
+  })
 })
 
 describe('protocol sentinels', () => {