fix(tui): visually distinguish markdown table rows from prose (#15534)

Tables rendered through `<Md>` had no separator and no header weight,
so they read as a paragraph with extra whitespace.  This adds two tiny,
border-free changes that survive Ink's grapheme-approximate column
widths better than a full outline:

* Bold the header row, keeping the existing amber colour.
* Insert a dim `─`-dashed rule between the header and body rows.

We deliberately stay away from a full outline — column widths are
measured via `stripInlineMarkup(...).length`, which is grapheme-aware
but still off by a cell on East Asian wide characters and emoji-mid-
cell strings.  A header rule plus the existing 2-space column gap
gives the visual hierarchy the issue asks for without amplifying that
inaccuracy into a misaligned border.

Validation: `npm run type-check` clean, `npm test --run` 389/389.

ui-tui/src/components/markdown.tsx

@@ -1,5 +1,5 @@
 import { Box, Link, Text } from '@hermes/ink'
-import { memo, type ReactNode, useMemo } from 'react'
+import { Fragment, memo, type ReactNode, useMemo } from 'react'
 
 import { ensureEmojiPresentation } from '../lib/emoji.js'
 import { highlightLine, isHighlightable } from '../lib/syntax.js'
@@ -97,18 +97,33 @@ export const stripInlineMarkup = (v: string) =>
 const renderTable = (k: number, rows: string[][], t: Theme) => {
   const widths = rows[0]!.map((_, ci) => Math.max(...rows.map(r => stripInlineMarkup(r[ci] ?? '').length)))
 
+  // Thin divider under the header.  Without it tables look like prose
+  // with extra spacing because the header is just amber-coloured text
+  // (#15534).  We avoid full borders on purpose — column widths are
+  // grapheme-approximate so a real outline often misaligns; one dim
+  // dashed rule under row 0 plus tab-style column gaps reads cleanly
+  // on every terminal we tested.
+  const sep = widths.map(w => '─'.repeat(Math.max(1, w))).join('  ')
+
   return (
     <Box flexDirection="column" key={k} paddingLeft={2}>
       {rows.map((row, ri) => (
-        <Box key={ri}>
+        <Fragment key={ri}>
+          <Box>
             {widths.map((w, ci) => (
-            <Text color={ri === 0 ? t.color.amber : undefined} key={ci}>
+              <Text bold={ri === 0} color={ri === 0 ? t.color.amber : undefined} key={ci}>
                 <MdInline t={t} text={row[ci] ?? ''} />
                 {' '.repeat(Math.max(0, w - stripInlineMarkup(row[ci] ?? '').length))}
                 {ci < widths.length - 1 ? '  ' : ''}
               </Text>
             ))}
           </Box>
+          {ri === 0 && rows.length > 1 ? (
+            <Text color={t.color.dim} dimColor>
+              {sep}
+            </Text>
+          ) : null}
+        </Fragment>
       ))}
     </Box>
   )