fix(tui): route Ctrl+B to voice toggle, not composer input

When the user runs /voice and then presses Ctrl+B in the TUI, three
handlers collaborate to consume the chord and none of them dispatch
voice.record:

- isAction() is platform-aware — on macOS it requires Cmd (meta/super),
  so Ctrl+B fails the match in useInputHandlers and never triggers
  voiceStart/voiceStop.
- TextInput's Ctrl+B pass-through list doesn't include 'b', so the
  keystroke falls through to the wordMod backward-word branch on Linux
  and to the printable-char insertion branch on macOS — the latter is
  exactly what timmie reported ("enters a b into the tui").
- /voice emits "voice: on" with no hint, so the user has no way to
  know Ctrl+B is the recording toggle.

Introduces isVoiceToggleKey(key, ch) in lib/platform.ts that matches
raw Ctrl+B on every platform (mirrors tips.py and config.yaml's
voice.record_key default) and additionally accepts Cmd+B on macOS so
existing muscle memory keeps working. Wires it into useInputHandlers,
adds Ctrl+B to TextInput's pass-through list so the global handler
actually receives the chord, and appends "press Ctrl+B to record" to
the /voice on message.

Empirically verified with hermes --tui: Ctrl+B no longer leaks 'b'
into the composer and now dispatches the voice.record RPC (the
downstream ImportError for hermes_cli.voice is a separate upstream
bug — follow-up patch).

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

@@ -31,6 +31,36 @@ describe('platform action modifier', () => {
   })
 })
 
+describe('isVoiceToggleKey', () => {
+  it('matches raw Ctrl+B on macOS (doc-default across platforms)', async () => {
+    const { isVoiceToggleKey } = await importPlatform('darwin')
+
+    expect(isVoiceToggleKey({ ctrl: true, meta: false, super: false }, 'b')).toBe(true)
+    expect(isVoiceToggleKey({ ctrl: true, meta: false, super: false }, 'B')).toBe(true)
+  })
+
+  it('matches Cmd+B on macOS (preserve platform muscle memory)', async () => {
+    const { isVoiceToggleKey } = await importPlatform('darwin')
+
+    expect(isVoiceToggleKey({ ctrl: false, meta: true, super: false }, 'b')).toBe(true)
+    expect(isVoiceToggleKey({ ctrl: false, meta: false, super: true }, 'b')).toBe(true)
+  })
+
+  it('matches Ctrl+B on non-macOS platforms', async () => {
+    const { isVoiceToggleKey } = await importPlatform('linux')
+
+    expect(isVoiceToggleKey({ ctrl: true, meta: false, super: false }, 'b')).toBe(true)
+  })
+
+  it('does not match unmodified b or other Ctrl combos', async () => {
+    const { isVoiceToggleKey } = await importPlatform('darwin')
+
+    expect(isVoiceToggleKey({ ctrl: false, meta: false, super: false }, 'b')).toBe(false)
+    expect(isVoiceToggleKey({ ctrl: true, meta: false, super: false }, 'a')).toBe(false)
+    expect(isVoiceToggleKey({ ctrl: true, meta: false, super: false }, 'c')).toBe(false)
+  })
+})
+
 describe('isMacActionFallback', () => {
   it('routes raw Ctrl+K and Ctrl+W to readline kill-to-end / delete-word on macOS', async () => {
     const { isMacActionFallback } = await importPlatform('darwin')