feat(tui): per-language syntax highlighting in markdown code fences

Adds a minimal hand-rolled highlighter for ts/js/jsx/tsx, py, sh/bash, go, rust,
json, yaml, sql. Recognizes whole-line comments, single/double/backtick strings,
numbers, and per-language keyword sets. Unknown langs fall through to the current
plain rendering; the existing diff-specific colorization is preserved.

Closes the §8 "Markdown syntax highlighting is missing (only diff gets colored)"
finding from the TUI v2 audit without pulling in a highlighter library.

ui-tui/src/lib/syntax.ts

@@ -0,0 +1,117 @@
+import type { Theme } from '../theme.js'
+
+export type Token = [string, string]
+
+interface LangSpec {
+  comment: null | string
+  keywords: Set<string>
+}
+
+const KW = (s: string) => new Set(s.split(/\s+/).filter(Boolean))
+
+const TS = KW(`
+  abstract as async await break case catch class const continue debugger default delete do else enum export extends
+  false finally for from function get if implements import in instanceof interface is let new null of package private
+  protected public readonly return set static super switch this throw true try type typeof undefined var void while
+  with yield
+`)
+
+const PY = KW(`
+  False None True and as assert async await break class continue def del elif else except finally for from global if
+  import in is lambda nonlocal not or pass raise return try while with yield
+`)
+
+const SH = KW(`
+  if then else elif fi for in do done while until case esac function return break continue local export readonly
+  declare typeset
+`)
+
+const GO = KW(`
+  break case chan const continue default defer else fallthrough for func go goto if import interface map package range
+  return select struct switch type var nil true false
+`)
+
+const RUST = KW(`
+  as async await break const continue crate dyn else enum extern false fn for if impl in let loop match mod move mut
+  pub ref return self Self static struct super trait true type unsafe use where while yield
+`)
+
+const SQL = KW(`
+  select from where and or not in is null as by group order limit offset insert into values update set delete create
+  table drop alter add column primary key foreign references join left right inner outer on
+`)
+
+const LANGS: Record<string, LangSpec> = {
+  go: { comment: '//', keywords: GO },
+  json: { comment: null, keywords: KW('true false null') },
+  py: { comment: '#', keywords: PY },
+  rust: { comment: '//', keywords: RUST },
+  sh: { comment: '#', keywords: SH },
+  sql: { comment: '--', keywords: SQL },
+  ts: { comment: '//', keywords: TS },
+  yaml: { comment: '#', keywords: KW('true false null yes no on off') }
+}
+
+const ALIAS: Record<string, string> = {
+  bash: 'sh',
+  javascript: 'ts',
+  js: 'ts',
+  jsx: 'ts',
+  python: 'py',
+  rs: 'rust',
+  shell: 'sh',
+  tsx: 'ts',
+  typescript: 'ts',
+  yml: 'yaml',
+  zsh: 'sh'
+}
+
+const resolve = (lang: string): LangSpec | null => LANGS[ALIAS[lang] ?? lang] ?? null
+
+export const isHighlightable = (lang: string): boolean => resolve(lang) !== null
+
+const TOKEN_RE = /'(?:[^'\\]|\\.)*'|"(?:[^"\\]|\\.)*"|`(?:[^`\\]|\\.)*`|\b\d+(?:\.\d+)?\b|[A-Za-z_$][\w$]*/g
+
+export function highlightLine(line: string, lang: string, t: Theme): Token[] {
+  const spec = resolve(lang)
+
+  if (!spec) {
+    return [['', line]]
+  }
+
+  if (spec.comment && line.trimStart().startsWith(spec.comment)) {
+    return [[t.color.dim, line]]
+  }
+
+  const tokens: Token[] = []
+  let last = 0
+
+  for (const m of line.matchAll(TOKEN_RE)) {
+    const start = m.index ?? 0
+
+    if (start > last) {
+      tokens.push(['', line.slice(last, start)])
+    }
+
+    const tok = m[0]
+    const ch = tok[0]!
+
+    if (ch === '"' || ch === "'" || ch === '`') {
+      tokens.push([t.color.amber, tok])
+    } else if (ch >= '0' && ch <= '9') {
+      tokens.push([t.color.cornsilk, tok])
+    } else if (spec.keywords.has(tok)) {
+      tokens.push([t.color.bronze, tok])
+    } else {
+      tokens.push(['', tok])
+    }
+
+    last = start + tok.length
+  }
+
+  if (last < line.length) {
+    tokens.push(['', line.slice(last)])
+  }
+
+  return tokens
+}